SURVEYING-I BTECH 3RD SEMESTER CIVIL ENGINEERING. (MODULE-1 to MODULE-IV) [SUB CODE-PCI3I102] Dr. Bhagirathi Tripathy. Asst.

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1 SURVEYING-I BTECH 3RD SEMESTER CIVIL ENGINEERING (MODULE- to MODULE-IV) [SUB CODE-PCI3I0] Dr. Bhagirathi Tripathy Asst. Professor Civil Engineering Department IGIT, Sarang, Parjang, Dhenkanal, Odisha-75946

2 SURVEYING Theory L/T (Hours per week): 3/0, Credit: 3 Module I (0 classes) ( Page to 80) Linear measurement and chain survey: Use of chains and tapes for measurement of correct length of lines, direct and indirect ranging, chaining along sloping ground. Obstacle in chaining, errors and their elimination. Compass surveying: Use of prismatic compass, temporary adjustment, bearing of a line, local attractions, correction of bearing Module II (8 classes) ( Page 8 to 0) Levelling: Use of dumpy level and levelling staff.temporary and Permanent adjustment of dumpy level, Reduction of levels by height of instrument and rise and fall method. Curvature and refraction error, sensitiveness of level tube, reciprocal levelling, leveling difficulties and common errors, Automatic and Electronic or Digital levels Module III (0 classes) ( Page 0 to 34) Contouring: Contour interval and horizontal equivalent, characteristics of contours,methods of contouring- different and indirect method, contour gradient Theodolite Survey: Use of theodolite, temporary adjustment, measuring horizontal and vertical angles, theodolite traversing Module IV (8 classes) ( Page 35 TO 48) Modern Surveying Instruments Electromagnetic Spectrum, Radar, Electronic Distance Measurement, EDM Equipment, Corrections to measurement, Digital Theodolite, Total Stations, Introduction to Remote Sensing and GIS Text Books.Surveying &Levelling. Vol I by T.P.Kanethar&S.V.Kulkarni, Pune VidyarthiGrihaPrakashan. Surveying and Leveling by R. Subramanian, Oxford University Press 3.Surveying-Vol.I, by B.C. Punmia, Laxmi Publications Reference Books.Surveying Vol - by R Agor, Khanna Publishers. A Textbook of Surveying, C. Venkatramaiah, Universities Press3. Surveying AndLevelling, N.N. Basak, McGraw - Hill Education

3 SURVEYING-I MODULE-I Linear measurement and chain survey: Use of various types of chains and tapes, measurement of correct length of lines, direct and indirect ranging, chaining along sloping ground. Obstacle in chaining, errors and their elimination. Compass surveying: Use of prismatic compass, temporary adjustment, bearing of a line, local attractions, correction of bearing Plane table surveying: Methods of plane tabling, radiations, intersection, traversing and resection, two point and three point problem. Adjustment and common error in plane table survey. 3

4 SURVEYING It is the art of determining the relative positions of different object on the surface of the earth by measuring the horizontal distance between them and by preparing a map to any suitable scale. Thus, in this process, the measurements are taken only in the horizontal plane. LEVELLING Leveling is the art of determining the relative vertical distance of different points on the surface of earth. Hence, in leveling, the measurements are taken only in the vertical plane. What is done in Chain Surveying? In Chain Surveying, only linear measurements are made i.e. no angular measurements are made. Triangle is the only figure that can be plotted with only measurement of sides to enclose an area. Hence, in chain survey area to be plotted should be covered with a network of triangles. Therefore, chain surveying is also known as Triangulation. Chain survey is the simplest method of surveying. In this survey only measurements are taken in the field, and the rest work, such as plotting calculation etc. are done in the office. This is most suitable adapted to small plane areas with very few details. If carefully done, it gives quite accurate results. Objectives, Use & Principles of Surveying Objectivess of surveying:. The primary objective of survey is the preparation of plan of a estate or buildings, roads, railways, pipelines, canals, etc. Or to measure area of field, state, nation.. Objectives of geodetic surveying is to determine precise positions on the surface of the earth of widely distant points. 4

5 Uses of Surveying:. To prepare a topographical map which shows the hills, valley, rivers, villages, town, etc, of a country.. To prepare a cadastral map showing the boundaries of fields houses, and other properties. 3. To prepare an engineering map to show details like roads, railways, canals, etc. 4. To prepare military map showing roads and railways, communication with different parts of country. 5. To prepare contour map and to determine capacity of a reservoirs and ton find the best possible route for roads, railways etc. 6. To prepare archeological map including places where ancient relics exist. 7. To prepare a geological map showing areas including underground resources. PRINCIPLE OF SURVEYING Fundamental principles.. To work from the whole to the part. Control points: - triangulation of traversing. Triangulation divided into large triangle. Triangles- subdivided in to small triangles To control and localize minor errors. On the other hand It we work from the part of the whole; small errors are magnified & uncontrollable at the end.. To fix the position of new stations by at least two independent process. The stations are fixed from points already fixed by Linear measurement or Angular measurements or B Both the linear and angular measurements.e.g. Chain surveying- main lines & stations points are checked by means of check or tie lines. 5

6 Types of Surveying [Classification]: A. Primary Classification or Primary Division :. Plane surveying and. Geodetic surveying. Plane Surveying : The shape of the earth is spherical. Thus the surface is obviously curved. But in plane surveying the curvature of earth is not taken into account. This is because plane surveying is carried out over a small area, so the surface of the earth is considered as a plane. The degree of accuracy required in this type of surveying is completely low. Plane surveying is done on an area of less than 50km.. Geodetic surveying : In geodetic surveying the curvature of the earth is taken into consideration. It is extended over a large area greater than 50km. The line joining any two points considered as a curved line. Very refined methods and instruments are used in this type of surveying. IN this method very high precision or accuracy is required. B. Secondary classification: Survey can be classified on different bases.. Based on instrument: A. Chain Survey B. Compass survey C. Plane Table survey D. Theodolite survey E. Tachometric Survey F. Photographic survey 6

7 . Based on methods: A. Triangulation Survey B. Traverse Survey 3. Based on Objects: A. Geological survey B. Mine survey C. Archeological Survey D. Military survey 4. Based on nature of field A. Land Survey B. Marine survey C. Astronomical survey Again Land Survey is classified into following Classes:. Topographical Survey To determine natural features of a country such as valleys, rivers and artificial features such as road, railways, etc.. Cadastral Survey: To determine boundaries of field, estate 3. City survey: To locate premises, streets, water supply and drainage systems 7

8 4. Engineering survey: To collect detailed data for the design for of projects involving roads, railways, etc Engineering surveys are sub divided into:. Reconnaissance Survey. Preliminary Survey 3. Location Survey Various methods used for linear measurements may be grouped as: (i) Approximate (ii) Using chain or tape (iii) By optical means and (iv) Using electromagnetic distance measurement instruments... Approximates Methods of Linear Measurements These methods are used in reconnaissance survey or to detect major mistakes committed while measuring with better methods. On smooth roads they can give results within per cent error. These approximate measurements may be by: (i) Pacing (ii) Using passometer (iii) using pedometer (iv) using odometer or by (v) using speedometer. (i) Pacing: In this method surveyor walks along the line to be measured and counts the number of steps. Then the distance measured is equal to number of steps average length of a step. Average length of a step can be found by walking along a known length. A normal man takes a step of length 0.75 m to 0.8 m. 8

9 (ii) Using Passometer: A passometer is a watch-like instrument which is carried vertically in the pocket of shirt or tied to a leg. It records number of steps taken. Thus the problem of counting number of steps is eliminated in this approximate method of linear measurement. (iii) Using Pedometer: This instrument is similar to passometer but it can record the distance instead of number of steps. In this, zero setting and setting of step length is made before walking. (iv) Odometer: This instrument is attached to the wheel of a cycle or other vehicle. It records the number of revolutions made by the wheel. Knowing the circumference of the wheel, the distance travelled may be found. (v) Speedometer: Odometer calibrated to give distance directly is called speedometer. This is to be used for particular vehicle only. All automobiles are provided with speedometers. By running the vehicle along the line to be measured distance can be found. Instruments used in Surveying Instruments Used for Measuring Distance:. Chain a. Metric chain b. Steel band chain c. Günter s Chain d. Revenue Chain e. Engineers chain. Arrows (chain pins) 3. Tapes a. Cloth or linen Tape b. Metric Woven Metallic Tape c. Metric steel Tape d. Invar tape. e. Synthetic Tape. 9

10 4.Wooden pegs. 5. Ranging Road. 6. Ranging Poles. 7. Offset Rod. 8. Laths 9. Whites 0. Plumb Bob. Chain The chain is composed of 00 or 50 pieces of galvanized mild steel wire 4mm in diameter called links. The end point of each link are bent into a loop and connected together by means of three oval rings. The ends of the chain are provided with brass handles for dragging the chain on the ground. The length of link is the distance between the centers of the two consecutive middle rings. The end links includes the handles. Metallic tags or indicators are fixed at various distinctive of the chain to facilitate quick reading. a. Metric surveying chains: The chains are made in lengths of 0 and 30 meters. To enable the reading of factious of a chain, tallies (tags) are fixed at every five meter length and small brass rings are provided at every meter length. To facilitate holding of the arrows in position with the handle, a groove is cut on the out side surface of the handle. The handle joints are flexible. the tallies used for marking the distances in a metric chain are marked with letters Me and m. 0

11 b. Steel Band Chain: It consists of a ribbon of steel with bras handle at each end. It is 0 or 30long and 6 mm wide. It is wound on an open steel cross or on the metal reel in a closed case. The graduations are etched as meters decimeters, centimeters on one side and 0. m links on the other. Brass tallies are fixed at every 5 m length of the band. c. Günter s Chain: It is 66 fit long and is divided into 00 links. Each link is 0.66 ft long. It is very convenient for measuring distance in miles and furlongs. Also for measuring area and when the units of area is an acre d. Revenue Chain: It is commonly used for measuring fields in cadastral survey. It is 33 ft long and divided into 6 links. Each link is.065 ft long. e. Engineer s chain: It is 00 ft long and it is divided into 00 links. Each link is ft in a length. Used in all Engineering surveys.

12 . Arrows (chain pins): They are also called as marking or chaining pins and are used to mark the end of chain during the process of chaining. They are made up of good quality hardened and tempered steel wire of 4mm in diameter. The arrows are made 400 mm in length. They are pointed at one end of inserting in to the ground. The other end is in to a ring. 3. Tapes: a. Cloth or Linen Tape: Used for taking subsidiary measurements, such as offset. It is very light and handy. It is easily affected by damp. If wet it shrinks. It stretches easily and likely to twist.

13 b. Metric Woven Metallic Tape: They are available in, 0, 30, and 50 meters. The tape is made of yarn and metal wire. A metal ring is attached to the outer end of tapes. The length of the tape includes the metal ring. At every centimeter a black line 8 to 0 mm in height is drown. Every 5 centimeters is marked with an arrow in black. Every decimeter and meter is marked with a back line extending over the full width of the tape/ the graduation marks at every decimeter and meter are numbered with black and red figures, respectively. c. Metric Steel Tape: Tape is available in,, 0, 30, and 50 meters. The tape is of steel or stainless steel. The outer end is provided with a ring. The length of the tape includes the metal ring. The tape is marked with a line at every five millimeters, centimeters, decimeters, and meter. Every decimeter and meter shall be marked with Hindu Arabic numerals in bold. When the button release devised is pressed, the tape automatically rewind in to the case. d. Invar Tape: For highest precision work the invar tape in used. It is made of an alloy of steel and nickel (36%).It is 6 mm wide and may be obtained in length of 30m and 00m. It is not calibrated through its length but has terminal lines. Each terminal division has ten mm division. It is very expensive. e. Synthetic Tape: The tapes are manufactured of glass glass fiber having PVC coating. They are graduated every 0 mm and figured every 00 mm. Meter, figures are shown in red. They are convenient for measuring shorts lengths. 3

14 Instruments for marking stations:. Wooden Pegs: These are used to mark the positions. They are made of hard timber and tapered at one end. They are usually,.5 cm square and 5 cm long. But in soft ground 40 to 60 cm long and 4 to 5 cm square is suitable. They should be driven in the ground with about 4 cm lengths, projecting above the ground.. Ranging rods: Used for making the positions of stations and for ranging. They are made of seasoned timber of teak, blue pine, sisov or deodar. They are circular or octagonal in cross section of 3 cm diameter. Lower shoe is 5 cm long. They are made in two sizes as meters and 3 meters and are divided in to equal parts each 0. m long. They are painted alternatively black and white or red and white. Now a day instead of timber, mild steel hallo pipes are used. 3. Ranging Poles: Similar to the ranging rods but are heavier, they vary in length from 4 m to 6 m or more. Used in the case of very long lines. 4. Offset Rod: Similar to the ranging rod, they are Offset usually 33.m long androd: is divided into parts each 0. m length. Top is an provided with an open ring for puling or pushing the chain through a hedge. It has two short narrow vertical slots. It is used for aligning short offsets. 4

15 Similar to the ranging rod, they are usually 3 m long and is divided into parts each 0. m length. Top is an provided with an open ring for puling or pushing the chain through a hedge. It has two short narrow vertical slots. It is used for aligning short offsets. 5. Laths: Useful for ranging long lines, also used over uneven ground where the ranging rod is not visible due to obstructions, they are light, cheap, being white; they are easily visible at a great distance. Unusually.0m long 6. Whites: When the ranging rod is not available or insufficient, whites are used. These are thin strip of bamboo and 40 cm to m in length. One end is sharp and the other end is split for inserting pieces of white papers. They are also useful for temporary marking of counter points. 7. Plumb Bob: The plumb bob is required when measuring the distance along slopes in order to transfer points to the ground. It is also used for testing the verticality of ranging poles. Technique of unfolding and folding of a metric chain. UNFOLDING: Remove the strap of the folded chain and take both the handles in the left hand and hold the remaining portion of the chain in the right hand. Holding both the handles in the left hand, throw the remaining portion o f the chain in the forward direction on the ground. Now the follower stands at the starting station by holding one handle and directs the leader to move forward by holding the other handle until the chain is fully stretched 5

16 FOLDING: Bring the two handles together on the ground by pulling the chain at the center. Commencing from the center two pairs of links are taken at a time with the right hand and placed alternatively in both directions in the left hand. When the chain is completely folded the two brass handles will appear at the top. Now tie the chain with leather strap. Technical terms used STATION- It is a point of importance at the beginning or at the end of a survey line. MAIN STATION- These are the stations at the beginning or at the end of lines forming main skeleton. SUBSIDIARY OR TIE STATIONS- These are the stations selected on main lines to run auxiliary/secondary lines for the purpose of locating interior details. BASE LINE- It is the most important line and is the longest line. Main framework of survey lines are built on it. DETAIL LINE- If the important objects are far away from the main lines, the offset readings are too large, which results into inaccuracies and time-consuming in the field work. In such cases the secondary lines are run by selecting stations on main lines. CHECK LINES- These are the lines connecting main station to a subsidiary station on the opposite side or connecting two subsidiary stations on the sides of main lines. These lines are also known as PROOF LINES. 6

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18 Selection of Stations A station selected should be visible from at least two more stations. If possible should have one or two base lines which run on level ground and through the middle of the area. Main frame should have as few lines as possible. All triangles should be well-conditioned. Each triangle should have at least one check line. Subsidiary stations should be selected such that offsets to main objects from subsidiary lines are as short as possible. Avoid obstacles to ranging and chaining. As far as possible survey lines should be on the level ground. Sides of the larger triangles pass as close to boundary lines as possible. They should be almost parallel to the boundary. Trespassing and frequent crossing of the roads should be avoided. Lateral measurements to chain line for locating ground features are known as Offsets. There are two types of offsets used in chain surveying viz. PERPENDICULAR OFFSET and OBLIQUE OFFSET. In case of perpendicular offset, foot of the perpendicular on chain line is found from the object and the surveyor notes down offset distance and the chainage of foot of the perpendicular. In case of oblique offset, the distance of the object from two convenient points on the chain lines are measured and noted down. 8

19 Setting out Perpendicular Offsets Perpendicular offsets may be set by Swinging Using cross staffs Using optical square Using prism square Perpendicular offset by Swinging The leader takes the end of the tape and holds it on the object. The follower swings the tape on the chain line and finds the shortest distance of the object from the chain line. Since the perpendicular distance is the shortest distance of a point from a line, it is noted as the perpendicular distance. The follower reads the corresponding chainage and the offset length. The recorder records it in the field book. Prism Square 9

20 It works on the principle as the optical square. In this case, a prism with angle between reflecting surfaces of 45 degrees is used as shown Field Work Reconnaissance- Reconnaissance of the area to be surveyed has to be undertaken first. This identifies key features of the area the survey are to be located and determines the kind of equipment needed to be carried to complete the survey. Equipment- Generally the following equipment will be required. A chain with at least 0 arrows, a metallic or steel tape, a dozen of ranging rods, an offset rod, pegs, a plumb bob, etc. Marking stations- Survey stations should be marked on the ground as per the plan prepared. Chaining and locating details- The survey lines are then measured accurately, starting from the base line. The details are located by taking offset at right angle or oblique offsets. Plotting a Chain Survey (Office work)after carrying field work next step in surveying is plotting to get plan of the area surveyed. It is carried out by the surveyor himself or the assistance of the draftsmen may be obtained. Steps involved in plotting are as follows: SCALE: Depending upon the are in the field and area of drawing sheet scale is decided. Normally, it is decided before the commencement of survey itself ORIENTATION: Skeleton of the network of triangles should be drawn to a scale on a tracing sheet and the orientation of the plan on drawing sheet be decided. After the orientation is decided using the skeleton diagram on tracing sheet base line and stations are pricked. 0

21 DRAWING NETWORK OF TRIANGLES: First base line is drawn to the scale. By intersection other stations are fixed and main triangles are drawn. The network of triangles is checked using check lines. PLOTTING OFFSETS: Any one of the following methods may be used Mark points along chain line and draw perpendicular with set square. If oblique offsets are taken, arcs are drawn from the respective positions on chain line and the position of the objects fixed up. Main scale and offset scale may be used. Main scale is kept along direction of chain line and offset scale gives the perpendiculars to it. OTHER DETAILS: If readings are taken to only one or two faces of buildings, using overall dimension outline of the building may be completed. Graphical scale should be plotted so that even if the paper shrinks, correct measurements may be obtained without calculations. Conventional Colours Roads- Burnt Sienna/Brown. Buildings- Light Grey. Compound wall- Indigo.

22 Water- Borders edged with Prussian blue. Trees- Green. Conventional Symbols Booking field notes A field book is used for noting the readings when the survey is carried out. The field book is about.5 cm long and.5 cm wide and opens length wise. Generally single or double red lines are drawn in the middle. Chainage lengths are noted in between the two red lines and the space on either side is used for writing the offset lengths and for drawing rough sketches of figures. The booking is commenced from the bottom of the page and work upward. The initial and final chainage should be marked with a symbol Δ and capital letters like A, B, C etc., should be used to mark the stations. While noting the readings, the surveyor should face the direction of chaining and offsets should be entered on the left or right side as they are situated. The conventional symbols (Fig.4) should be used for recording different features. Each new chain line should be started from a new page. One sample page of a field book is shown in figure. At the commencement of the line in the book is written: the name (or) numbers of the survey line, the name (or) number of station, the symbol perpendicular donating the station, tie (or) subsidiary station should be indicated by a circle (or) a oval round their chainage figures, all around the chain line are entered in the central column and offset retain opposite them on the right (or) left of the column according as they are right (or) left of the survey line.

23 Field book entry Plotting procedure of chain survey After the survey is completed, the recorded information is taken to the office and they are plotted on a drawing sheet using a suitable scale. While plotting a chain survey, the following steps are to be followed: (i) Depending upon the area covered in the survey and its importance, a suitable scale is chosen. Boundary lines are drawn leaving a suitable margin all around. (ii) The base line which is the mainline in the survey should be suitably located in the map to accommodate the whole plotting easily in the drawing sheet. The base line should be most accurately plotted. (iii) The intermediate stations are marked on the base line and the frame work of triangles is completed. (iv) Chainage lengths are measured along the chain lines for various offsets and points are marked. From these points perpendiculars of suitable lengths are drawn to locate the offsets. (v) The accuracy of plotted frame work may be checked by means of check and tie lines. (vi) The field book should be kept side by side in the same direction as the survey proceeded in the field parallel to the chain line to be plotted. (vii) For drawing different objects, conventional symbols should be used. (viii) The title of the survey, name of the surveyor, date etc., should be written at the right hand bottom corner. The scale is drawn below the map. 3

24 5.7 Drawing instruments required The following drawing instruments are generally used in drawing office works.. Drawing table (size.4.m). A drawing board of good quality ( m) 3. The square with ebony edge (lengths vary from m) 4. Set squares required for drawing parallel and perpendicular lines. 5. An instrument box 6. Protractor 7. Four set of French curves required for drawing irregular and curved figures. 8. A standard straight edge of m long for drawing long lines. Traversing with chain and tape only Though triangulation is the basic principle of chain surveying, it is possible to go for traversing using only chain and tape. Traversing is the survey which is conducted along desired lines by measuring the length and the direction of survey lines. CLOSED TRAVERSE- When the lines form a circuit which starts from a line and after covering an area ends at starting point. OPEN TRAVERSE- If the starting point of survey and ending point are different. Closed traverse- lakes and building surveys. Open traverse- road and canal surveys. 4

25 RANGINGIt is the process establishing a new station between two intermediate stations When a survey line is longer than a chain length, it is necessary to align intermediate points on chain line so that the measurements are along the line. The process of locating intermediate points on survey line is known as ranging. There are two methods of ranging viz., direct ranging and reciprocal ranging. Direct Ranging If the first and last points are inter visible this method is possible. Figure below shows the inter visible stations A and B in which an intermediate point C is to be located. Point C is selected at a distance slightly less than a chain length. At points A and B ranging rods are fixed. The assistant holds another ranging rod near C. Surveyor positions himself approximately m behind station A and looking along line AB directs the assistant to move at right angles to the line AB till he aligns the ranging rod along AB. Then surveyor instructs the assistant to mark that point and stretch the chain along AC. 5

26 Indirect or Reciprocal Levelling Due to intervening ground, if the ranging rod at B is not visible from station A, reciprocal ranging may be resorted. Figure below shows this scheme of ranging. It needs two assistants one at point M and another at point N, where from those points both station A and station B are visible. It needs one surveyor at A and another at B. To start with M and N are approximately selected, say M and N. Then surveyor near end A ranges person near M to position M such that AMN are in a line. Then surveyor at B directs person at N, to move to N such that BNM are in a line. The process is repeated till AMNB are in a line. OBSTACLE IN CHAININGi) Chaining is free, vision is obstructed, eg. raising ground (or) a hill intervening. ii) Chaining obstructed but vision free, eg. pond, river, plantations, and tank. iii) Both chain and vision are obstructed, eg. buildings. 6

27 a) CHAINING IS FREE BUT VISSION IS OBSTRUCTED. Procedure for case-: Let A and B are the two stations across a hill. Ranging rods are placed at one of them is not visible from other. The following may be followed for ranging. ) As shown in fig below select two intermediate points C and D such that ranging rods at B and D are visible from C and ranging rods at A and C are visible from D. Also A, C, D, B should be nearly as possible in a straight line. ) The person at C looks towards B and directs the man at D to fix his ranging rod in a manner such that C, D, B are in one straight line. 3) Now the person at D looks towards the ranging rod at A and directs the man at C to fix his ranging rod at a place such that A, C and D are in one straight line. 4) Steps and 3 above are repeated till the person at C finds C, D,. B to form a straight line and simultaneously, the person at D finds A, C, D also to form a straight line, then all the four points A, C, D, and B are lie in straight line. Procedure for case : This case occurs, when it desired to run a line across a wooded field, the trees and under-bush preventing the fixing of intermediate stations. In such a case, the method of random line is the most suitable. 7

28 As shown in fig. 9 (b), let AB be the line whose length is required. From A, run a line (AB), called a random line in any convenient direction, but as nearly towards B as can be judged and continue until the point B is visible from B. Chain the line to B, where BB is perpendicular to AB and measure BB, then AB =AB+BB b)vision IS FREE BUT CHAINING IS OBSTRUCTED. This problem is to find out the distance between two convenient points on the chain line on either side of the obstruction. There are two cases: Case-: In which, it is possible to chain round the obstruction. Ex: A thorny hedge, a pond, a bend in the river. Case-: In which, it is not possible to chain round the obstruction. Ex: River Procedure for Case-: Select two convenient points A and B on the chain line PR and on either side of the obstruction (Fig.0). Then, erect equal perpendiculars AC and BD and measure the length CD. Then AB=CD. Procedure for Case-: Select two points A and B on the chain line PR on opposite banks of river (Fig.). Set out a perpendicular AD and bisect it at C. At D, erect a perpendicular DE and mark the point E in line with C and B. Measure DE, since the triangles ABC and CED are similar, then AB=DE. 8

29 Chaining obstructed but vision free (Case-) Chaining obstructed but vision free (Case-) iii) Both chaining and vision both obstructed: In this case the problem consists in prolonging the line beyond the obstruction and determining the distance across it. A building is a typical example of this class of obstruction. Procedure: Choose two points A and B on the chain line PR (Figure). At A and B, erect perpendiculars AE and BF of equal lengths. Check the diagonals BE and AF, which should be equal and also EF, should be equal to AB. Prolong the line EF past the obstruction and select two points G and H on it. At G and H, set out perpendiculars GC and HD are equal in length to AE. The points C and D are obviously on the chain line PR and BC=FG. Great care must be taken in setting out perpendiculars and to see that their lengths are exactly equal. Both chaining and vision are obstructed 9

30 Uniformly Sloping Ground If the ground slopes by more than about 3, this must be allowed for in the survey. The measured distances are thus slant distances and must be corrected to true horizontal distances. This requires that the vertical angle between the stations is known For an approximate survey, it may be sufficient to step up or downhill using a series of horizontal and vertical lines If the drop is measured at the same time, some estimate of the slope profile can be obtained Stepping method- 30

31 If stepping is not appropriate, more sophisticated methods must be used to measure the slant distance and the vertical angle simultaneously Requires optical sighting equipment: usually either a clinometer, Abney level or theodolite Correcting for horizontal distance: the hypotenusal allowance x Correction Factor= xy-yz =xy(-cos a) h a y z Testing of a chain It is always necessary to check the length of chain before commencing each day s work and at frequent intervals; otherwise the measurement will become unreliable. Before testing the chain, the surveyor should see that the links and rings are free from mud that there are no links or bent links. The chain is tested by comparing it with (i ) the chain standard (standard chain length) (ii) with the steel tape which should be kept in the surveyor s office for this sole purpose. If these are not available at hand, a test gauge may be established by driving two stout pegs in the required distance apart (0 m or 30m) and inserting nails into their tops to mark exact distance. It is advisable to have a permanent test gauge established in close proximity to the surveyors office. During the first use, the links become bent and, consequently the chain is shortened. It is also shortened by mud clogging the links when working over muddy ground. On the other hand, it gets elongated due to wear of many wearing surfaces, stretching of the links and joints, and opening out of the small rings and rough handling in pulling it through hedges and fences. 3

32 Adjusting the chain If the chain is found to be too long, it may be adjusted by i) Closing up the joints of the connecting rings (that may be opened out) ii) Hammering back to the shape of the elongated rings iii) Replacing some of the worn out rings with new ones iv) Removing one or more of the small rings. If the chain is found to be too short, it may be adjusted by i) Straightening any bent links ii) Flattening some of the small connecting rings iii) Replacing some of the worn out rings with new ones iv) Replacing a few of the rings by those of the larger size v) Inserting new rings as required Errors in chaining The errors that occur in chaining are classified as i.) compensating and ii) cumulative. These errors may be due to variation in temperature, defects in construction and personal defects in vision. Compensating errors: are those which are liable to occur in either direction and hence tend to compensate i.e. they are not likely to make the apparent result too large or too small. Compensating errors are caused due to incorrect holding of the chain, fractional part of the chain may not be correct and during stepping operation, crude method of plumbing is adopted Cumulative errors: are those which occur in the same direction and tend to add up or accumulate i. e., either to make the apparent measurement always too long or too short. (i) Positive errors - These errors makes the measured length more than actual length. (ii) Negative errors - making the measured length less than the actual. ERRORS AND OBSTACLES IN CHAINING Errors in Chaining Any field surveying including chain surveying has many errors including observational errors affecting the accuracies of measurements and mapping. It is essential to identify, rectify and adjust these errors before the results of surveying can be used for any engineering applications. These errors can be broadly classified as (a) Instrumental Errors (b) Observational errors 3

33 Instrumental Errors Instrumental Errors are caused by imperfections in instruments, wear andtear of instruments due to continuous use and their rough handling. Instrument are thus required to be tested for accuracy, adjusted and calibrated at frequent interval to ensure that the results of surveying exercises are well within the prescribed limits of accuracy and tolerances. Observational errors Observational errors are introduced because of involvement of human factor in surveying process. It should be accepted that whenever a human element is involved the process result will be influenced by the attitude, efficiency and perception of individual human being in a subjective manner.these can be avoided by proper training of surveyors, prescribing adequate and suitable precautions to be undertaken in each observational and measurement process and specifying proper and detail method statements for performing each operation of the process. Both these type of errorsi.e instrumental and observational can be further classified into (a) Gross errors (b) Systematic error (c) Accidental or random errors Gross errors:gross errors or mistakes are blunders that occur due to inexperience or carelessness on the part of the surveyor. In chain surveying these could be due to Displacement or loss of pegs or arrows provided to identify and fix the location of various type of stations and places of interest. Reading the chain or tape in a wrong manner or using an instrument in a wrong way. Wrong recording of measurements in the record book e.g field book. There is no room for gross erros or blunders in the surveying process. If gross errors are detected, the entire surveying process and measurements are required to be repeated afresh, resulting in substantial loss of time and resources. Such errors can be avoided by proper training and testing of surveyors adopting standard procedures, even to the minute details and carrying out the survey works with utmost care. 33

34 Systematic errors Systemetic errors follow some specific pattern according to some mathematical or physical law. The error could be cumulative i.e occurring in the same direction and trends to accumulate affecting the accuracy of measurements to a great extent. In the context of chain surveying, these could be due to : (a) Erroneous length of chain or tape (positive or negative) (b) Erroneous ranging (c) Links in chain not straight ( local bends) due to rough handling or twisting of metallic tapes etc. (d) Non- horizontally of chain or tape over rough ground terrain (e) Sag in chain or tape, when it is stretched across a depression in ground (f) Variation in temperature and/or dampness and (g) Variation in pull applied during measurement. These errors could be identified and adjusted and can be modeled. Suitable corrections can be applaied to the measurements for obtaining greater accuracy. Following are some of the important corrections applaied to measurements using chain or tape. Correction for erroneous length chain or tape The chain surveying depends only linear measurement of distances. For traversing only the errors in distance measurements are of importance and significance. 34

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36 Random or accidental Errors Random and accidental errors can occur due to lack of perfection of human eye and or human behaviour. Even the best and efficient surveyor can have fatigue effect after working for longer duration in strenuous environment causing obseravationalerrors. The randomerrors cannot be eliminated entirely,whatever precautions are undertaken.these may, however,occur in either the direction and hence, tend to compensate and,thus,can be analysed with the help of probality theory.using suitable probability distribution functions., these errors can then be adjusted, distributed among various measurements and account for. Each surveying method or process can be assigneda reliability factor (or risk factor) for accuracy depending on the analysis of probality behaviour. EXAMPLE- A 30m chain was found to be 3cm too long after chaining 800m. The same chain was observed to be 5cm too long after chaining the total distance of 3600m. Assuming that the chain was correct at the commencement of work, find the true length of the total distance chained. Solution:(a) During Chaining from 0 to 800m: Initial length of chain at commencement of work = 30m Final length of chain at the end of chaining upto 800m = 30.3m Average true length of the chain during this exercise = ( )/ = True distance of measured distance 800m = (L /L) X 800 = (30.05/30.0)X 800=800.90m.(i) (b) During Measurementfrom 800 to 3600: 36

37 Initial length of chain = 30.3m Final length of chain = 30.05m Average true length of the chain during this mesurement = ( )/ = measured distance = =800m True measured distance = (30.04/30.00)X 800 = 80.40m (ii) So, Total true distance chained = = m EXAMPLE- 37

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40 CHAPTER- COMPASS SURVEYING. INTRODUCTION In chapter, we have learnt that the general objective of surveying is to determine the relative positions of distinctive features on ground (in the area under study) in order to prepare maps which can be used in future for various engineering applications, e.g. planning, designing and executing civil engineering projects. This, in general, will consist of measuring distances, both horizontal and vertical, and angles. In Unit, you have studied different methods of measuring horizontal distance along the survey lines and offset distances of ground features from survey lines. However, if the area to be surveyed is comparatively large it is rather difficult and inaccurate to obtain all the required distances by measuring horizontal distances only. In real life situations, there are many obstructions and difficulties in direct ranging the survey lines. It becomes very tedious and inconvenient, even impossible, to fix the directions of survey lines by linear horizontal measurements alone. These problems can be conveniently solved by making angular measurements. The directions of survey lines can be easily fixed by measuring the angles between two survey lines meeting at a station or angle of a line with reference to some fixed axis. The direction of a line relative to a given fixed axis (or meridian) is called its bearing. Traversing becomes quite convenient by carrying out the measurements along a series of interconnected survey lines either to form a closed or an open traverse. The lengths and offsets are measured by chain or tape while directions of survey lines are measured using compass or any angle measuring instrument. This process is termed as chain and compass survey. Objectives After studying this unit, you should be able to understand various important terms and instruments used in compass surveying, understand the procedure of compass surveying, measure bearing of survey lines and find the angle between these lines, and 40

41 know various types of error involved in compass surveying, their source and methods of correction.. DEFINITIONS AND IMPORTANT TERMS The instruments commonly used for angular measurements are the compass and theodolite. Before the study of instrument and procedures used for compass surveying, it is necessary to define some important terms commonly used in this context... Bearing The horizontal angle between the reference meridian and the survey line is termed as bearing of the survey line. Magnetic Bearing The magnetic needle of the compass always points towards the magnetic north-south (N-S) direction indicating earth s magnetic axis. Since this direction is same at all the places on the earth s surface, it is universally used as the reference direction. The angle made by survey line in a clockwise direction with reference to magnetic N-S line is termed as magnetic bearing of the o o line. The value of magnetic bearing ranges from 0 to 360. True Bearing The geographical north of earth is different from the magnetic north. Hence, the angle which the survey line makes with the true geographical north is termed as true bearing of the survey line. Arbitrary Bearing It is the horizontal angle which a survey line makes with any arbitrary meridian, which is any convenient direction towards a permanent and prominent mark or signal, such as a church spire or top of a chimney. Such bearings are used to determine the relative position of line in a small area. 4

42 Whole Circle Bearing (WCB) o The complete circle of angular measurement starts with north as 0 and ends at north at o o o 360. The bearing of line directly obtained by magnetic needle ranging from 0 to 360 is called whole circle bearing Reduced Bearing (RB) The more convenient way to comprehend the direction of a survey line is to represent the bearing on a quadrantal system. The angle is measured withrespect to N S line towards east or west The relationship between WCB and RB). Fore Bearing (FB) The angle measured in the direction of survey line from starting survey station to the next station is called fore bearing. if the bearing of line AB is measured from A towards B, it is known as forward bearing or fore bearing. Back Bearing (BB) It is the bearing of the survey line taken from the forward survey station to the preceding station from which the fore bearing was taken earlier. if the bearing of same line AB is measured from B towards A, it is known as backward bearing or back bearing. 4

43 Fore Bearing Back Bearing = 80º Magnetic Declination The direction of magnetic meridian varies from place to place across the globe. Hence, the bearings taken with reference to magnetic meridian of the survey lines will not represent true relative angles between them. The errors can be negligible for smaller area surveys but will be quite significant for large surveys particularly geodetic and astronomical surveys. A more accurate method of denoting the bearing of a survey line will be to obtain the true bearing of the line. The difference between true bearing and magnetic bearing of a survey line at a survey station is called magnetic declination of that place. Hence, the horizontal angle made by magnetic meridian and true meridian at a place is termed as magnetic declination of that place as depicted in Figure 3.. The magnetic declination could be East declination Figure 3.(a) or West declination as in Figure 3.(b) indicating whether magnetic North is toward East or towards West of true North. 43

44 Determination of True bearing and magnetic bearing (a) True bearing = magnetic bearing ± declination Use the positive sign when declination is east and negative sign when declination is west. Magnetic bearing = True bearing ± declination. Use the positive sign when declination is west and negative sign when declination is east. It may also be noted here that position of magnetic North may change even at a station due to several factors. Hence, it may be necessary to record the date of survey and time to obtain the true bearing of a survey line at a place. Astronomical observations are required to be taken to determine the direction of true North at a place and hence to obtain the true bearing of a survey line at that place. Magnetic bearing can be easily obtained for the same line by compass survey. 44

45 The difference between true bearing and magnetic bearing of the survey line so obtained will give the magnetic declination at that place, i.e. Magnetic Declination = (True Bearing Magnetic Bearing) The magnetic declination will be positive if magnetic meridian is towards east of true meridian and negative if magnetic meridian is westward of true meridian. Isogonic Lines If the points on the globe which have same magnetic declination at a point of time are joined, the imaginary lines so obtained are called isogonic lines. Agonic Lines These are imaginary lines constructed by joining the points at which the magnetic declination is zero, and hence have the same value of magnetic bearing and true bearings. 45

46 For reference to geodetic and other important surveys, isogonic charts are published by agencies like Survey of India, on which isogonic and agonic lines are drawn on earth maps. Magnetic Dip The magnetic bearing of a survey line at a place is obtained by using a magnetic compass. The needle of this compass will not remain horizontal due to magnetic influence of the earth. This deflection of the needle from the horizontal position is called dip of the needle. Apart from local effects due to presence of magnetic ores in ground or such other localised influences, the magnetic dip of the compass needle will vary from place to place on the surface of earth. It will be horizontal at equator, i.e. zero dip at a place equidistant from both the poles. The deviation from horizontal position will gradually increase as survey lines moves toward the poles. This dip will influence the accurate recording of the bearings. A sliding weight or an aluminium coil can be placed on the higher side of the needle to counter balance this dip and make the needle perfectly horizontal during bearing measurements..3 INSTRUMENTS AND PROCEDURES.3. Compass The compass essentially consists of a freely suspended magnetic needle mounted on a smooth pointed pivot. The needle can freely move over a graduated scale. Two slit vanes are provided on o the frame one as the object vane and other as eye vane placed at 80 to provide the line of sight. A tripod stand is provided on which the compass can be mounted and positioned over the survey station, while taking observations. A circular metal box, approximately 00 mm diameter, is used with a hardened steel pivot at the centre. The magnetic needle, graduated aluminium ring and vanes etc. are other parts of the compass. Design of these parts and their placement vary in different types of compass. The two types of compass prismatic compass and surveyors compass are currently used in practice. Prismatic Compass It is the commonly used compass for engineering surveys and is suitable for surveys where speed is more important than accuracy, for example, the preliminary surveys of road, railway line or 46

47 pipe line alignments and rough traversing etc. Figure 3.3 shows the different constituents of a prismatic compass in their final assembled form. The aluminum ring of prismatic compass has a magnetic needle marked with N-S along the o o diameter of the ring. The graduations are itched from 0 to 360 in clockwise direction with zero o marked at south end of needle and 80 at the north end (Figure 3.4(a)). The itching is marked in inverted fashion so that they are read in correct way when viewed through the reflecting prism. Each degree in graduation is divided into half to give a least count of 30'. The object vane has a vertical hair thin wire bisecting the object under observation. The observation vane (or eye vane) consists of a reflecting prism. Both the vanes are collapsible to be folded to lie on compass cover when not in use. A plane mirror is hinged to object vane to sight the object which is too high or too low to be sighted directly. The indication of mirror can be adjusted to facilitate this process. In case of sun glare, when making the measurements become difficult, sun screen of tinted glasses can be used by placing them in the line of sight between prism and object vane. 47

48 To dampen the oscillation of magnetic needle and providing stability to measurement process, a brake pin is provided on the side of the compass box. A lifting pin is also provided to lift the needle and to keep it pressed against glass cover when the object vane is folded and the compass is not in use. This prevents the pivot from excessive wear and tear..3. Procedure of Measuring Bearing with Prismatic Compass The procedure of measuring bearing with the compass is discussed in this section along with some related issues like compass traversing, local attraction and correction due to local attraction. Setting the Compass at Station The prismatic compass is required to be temporarily set over the station at which the bearing of survey line required to be measured. It is basically a two-step procedure. This is also called the temporary adjustments of compass. Centering The compass is set so that its centre lies exactly above the station under consideration. This is achieved by suspending a plumb bob from the centre hook provided. If the conical end of plumb bob lie exactly over the station (X is marked over station for accuracy), the compass is considered to be exactly centered. If not, the legs of the tripod are adjusted in position by moving one leg first and then simultaneously moving other two legs in perpendicular direction to first movement. Several trials can be needed for obtaining the correct centering of the compass. In real life situations, when plumb bob is not available, a small piece of stone or pebble can be taken, by holding this stone by fingers in line of centre of compass and allowing it to drop freely 48

49 on the station. If the stone falls on the top of peg then centering is correct, otherwise the adjustment of tripod is done as explained earlier. Levelling The compass is required to be levelled so that the aluminium ring is in horizontal plane and hence free to rotate on pivot. The levelling can be checked by a spirit level or by rolling a pin on compass box. If the round pin does not roll, the level is correct. If not levelled correctly, the level can be adjusted by moving the legs of tripod. Some instruments are provided with a ball and socket arrangement at box base to achieve rapid levelling till the graduated ring moves freely inside the compass box. Observing the Bearing Once the compass is centered over the station and levelled, the process of bearing measurement can start. Let AB be the survey line as shown in Figure 3.5(a), the bearing of which is required to be measured. The instrument is set at A and a ranging rod is fixed at B. The compass is turned so that line of sight is aligned in the direction of AB by making eye slit of observation vane, vertical hair of object vane and ranging rod at B in same horizontal line. Wait for oscillation of graduation ring to dampen, with the use of brake pin if necessary. The viewing prism is focused by moving it vertically with the help of focusing stud. The reading of the image of hair line as observed through prism is noted indicating the whole circle bearing of survey line. The process is repeated to check the repeatability of measurements. This bearing is called fore bearing of line AB. 49

50 Traversing The instrument is successively set at each station of the traverse and the fore bearing and back bearing of each line is taken and recorded in the field notebook. The observational errors in this survey tend to compensate as each bearing is observed independently. Distances between each survey stations are measured using a chain/tape. The offset points are located either by procedure followed in chain surveying or by angular measurement with compass. The bearings of survey lines in a traverse are observed in progressive way. The bearing recorded in the direction of progress of survey is called the fore bearing while the bearing of the same survey line from the end station (station B on line AB) is termed back bearing (Figure 3.(d)). o It can be noted that back bearing of a line is equal to its fore bearing ± 80. Plus sign is used o o when fore bearing is less than 80 and minus sign is used when it is more than

51 PRINCIPLE OF COMPASS SURVEYING The principle of compass surveying is traversing, which involves a series of connected lines. The magnetic bearings of the lines are measured by prismatic compass and distances of the lines are measured by chain. Such survey does not require the formation of a network triangles. Interior details are located by taking offsets from the main survey lines. Sometimes subsidiary lines may be taken for locating these details. Compass survey is recommended when:. A large area is to be surveyed.. The course of river or coast line is to be surveyed and 3. The area is crowed with many details and trangulation is not possible. Compass surveying is not recommended for areas where local attraction is suspected due to presence of magnetic substances like steel structurers, Iron ore deposits, electric cables conveying current and so on. Methods of traversing- 5

52 Compass traversing: Fore bearings and back bearings between the traverse leg are measured Theodolite traversing: Horizontal angles between the traverse legs are measured. The length of the traverse legs are measured by chain/tape or by stadia method. Plane table traversing: Plane table is set at every traverse station in clockwise and anticlockwise direction and the circuit is finally closed. During traversing the sides of the traverse are plotted according to any suitable scale.. Closed Traverse When a series of connected lines forms a closed circuit i.e when the finishing point coincides with the starting point of a survey, it is called a closed traverse. Here ABCDEA represents a closed traverse as shown in figure below. Closed traverse is suitable for survey of boundaries of ponds, forests, estates etc. Check on closed traverse: Sum of the measured interior angles (n-4) x 90 Sum of the measured exterior angles (n+4) x 90 The algebric sum of the deflection angles should be equal to 360. Right hand deflection is considered +ve, left hand deflection ve Check on linear measurement The lines should be measured once each on two different days (along opposite directions). Both measurement should tally. Linear measurement should also be taken by the stadia method. The measurement by chaining and stadia method should tally. 5

53 Checks on traverse: Open traversetaking cut-off lines: measured the bearings and lengths of cut off lines after plotting and tally with actual values. Taking an auxiliary point: Take P permanent point as auxiliary point measured bearings and lengths of P from each traverse point. If survey is accurate, while plotting all the measured bearing of P should meet at P. PRECAUTIONS AND ERRORS IN COMPASS SURVEY.5. Precautions While undertaking the compass traversing, following precautions should normally be observed. 53

54 Bearing If it is difficult to observe the location of the ranging rod at station B from compass set at station A for obtaining the bearing of survey line AB, locate an intermediate station C on line AB, which can be sighted from both stations A and B. The compass can then be set over the intermediate station C. When there is an optical obstruction in the line AB, a parallel line C D is set out by means of offsets as nearly as possible (Figure 3.9(b)) and get the bearing of the survey line. Freeing the Needle The magnetic needle has to rotate freely over the pivot to get accurate measurement of bearing of the survey line. Tap the compass box after the needle has come to rest. This helps in overcoming the pivot friction, if any. The cover glass may also have gathered static electric charge, when rubbed with cloth while dusting and thus attract and jam the magnetic needle of compass. The glass has to be discharged by applying moist finger on its surface. Damping The vibration of compass needle are damped by gently pressing the braking knob. To reduce vibrations and to minimize wear and tear of pivot point, the needle shall be released only when the compass is aligned approximately in the direction of magnetic meridian at site. 54

55 It is always advisable to take duplicate reading of the needle for each bearing measurements. After noting down the first reading the compass is rotated to displace the needle. Readjust the needle before taking the duplicate reading. This reduces observational errors Sources of Error Various errors observed during a compass survey can be broadly classified as Instrumental errors, Observational errors and External influences. Instrumental Errors These could be due to defective manufacture or due to damage to instrument during rough handling, transportation and use. For example, (a) The needle may not be perfectly straight or balanced. (b) Needle loosing its magnetic property. (c) The pivot may become blunt or bent. (d) The plane of sight loosing its verticality and/or twisted so that it is not passing through the centre of compass. 53 Compass Surveying (e) The graduated circle may loose its shape or horizontality. Observational Errors Even when the instrument is in perfect order, some errors may occur during bearing measurements. These can be due to (a) Setting and levelling inaccuracies, i.e. the compass center may not coincide the center point of survey station, or it may not be levelled accurately so that it does not lie in a horizontal plane. (b) Ranging inaccuracies, i.e. the ranging rods at other object stations may not be fixed in vertical position or these may not be perfectly bisected by line of sight. (c) Reading and recording inaccuracies, i.e. due to carelessness, the position of line of sight may either be not read properly or accurately or wrongly recorded in field notebook. 55

56 External Influences Perfect instruments and their perfect use may not make the measurements error free because of the following reasons : (a) Magnetic storms, sunspots, lunar perturbations or minor tremors in earth may cause irregular variations in bearing measurements. (b) Secular, annual and/or diurnal variations in declination affect the bearing accuracy due to variation in magnetic meridian. (c) The local attraction due to presence of iron ore in ground, or steel structures, electric lines etc. in the vicinity of survey stations Error Prevention Having observed various types of possible errors during compass surveying, the surveyor has to take adequate measures during actual use of instrument to minimize the effects of these errors. Some of these are given below. Ensure Horizontality of Needle and Scale If the needle is not horizontal even when the compass is levelled properly, a small coil of brass rider is used by sliding it on needle towards the higher end of needle. Proper adjustment of rider will make the needle and scale horizontal. Ensure that Pivot is Central to Scale Readings at both, North and South, end of needle are recorded. The difference shall be o exactly 80. Any deviation from this will indicate that either the needle is not straight or the pivot is bent. If the difference between N- and S-readings is constant for different o positions of compass, though it may not be 80, it will indicate that needle is not straight while pivot is in centre. The needle is carefully observed and straightened to remove this deviation. If the deviation is not same for different compass position, pivot bending is indicated. Pivot is bent and needle straightened to remove this error. 56

57 Ensure Verticality of Plane of Sight A plumb bob is suspended in front of the compass set in position and is observed through the instrument. The eye vane, the object vane and the string of plumb bob shall be in same vertical plane. Any deviation will indicate the loss of verticality of either the eye vane or object vane which are then adjusted accordingly. Closing Error When a closed traverse survey is conducted and the results plotted, it may be observed that traverse fails to close. Actual distance by which traverse fail to close is called the closing error. These could be either due to (a) error in measuring angles, or (b) error in measuring distances. All the included angles of the traverse are computed from the recorded bearings and aggregated. If the aggregated included angle is equal to (n 4) right angles, the angle measurements are correct provided there is no local attraction influence or observational error. Any difference will indicate error in angular measurement. If the closing error is large, the survey is rejected and repeated. If it is small, the error can be corrected by making small adjustments in bearings as explained in Section 3.3. under Adjustments and Corrections. Errors in Chaining The errors in chaining arries from thr followings Due to Erroneous length of chain or tape. Due to Bad Ranging Due to Careless holding and marking. Due to Bad straightening of the chain which results more length of a line. Non horizontality of the chain during measurement Sag in Chain during measurement bylifting condition measurement of chain. Variations in Temperature during chaining as the length of chain varies with temperature. Due to Variations in pull of the chain man. Due to Personal mistake. 57

58 Example Convert the following whole circle bearings to reduced bearings o (a) 4 58 o (b) 56 o (c) 9 47 o (d) Solution The conversion can be conveniently achieved with the help of sketches as shown below : o (a) WCB = 4 58 st The survey line lies in quadrant o Hence RB = WCB, i.e. RB = N 4 58 E o (b) WCB = 56 o In second quadrant RB = 80 WCB o o o = = S E 58

59 o (c) WCB = 9 47 In third quadrant o RB = WCB 80 o o o = = S W o (d) WCB = In fourth quadrant o RB = 360 WCB o o o = = N 3 6 W 59

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62 CHAPTER-3 PLANE TABLE SURVEYING INTRODUCTION The plane table is an instrument used for surveying by a graphical method in which the field work and plotting are done simultaneously. In plane table surveying, an unknown point of interest is established by measuring its directions from known points. The main advantage of plane tabling is that the topographic features to be mapped are in full view. Plane table surveying is most suitable for small and medium scale mapping. Objectives After studying this unit, you should be able to understand the basic principle of plane table surveying, its advantages and disadvantages, identify the equipment and accessories used in plane tabling, describe the adjustments of plane table, describe the procedure of setting up the plane table, understand the procedures of recording observations, understand various methods of resection, and describe the possible errors in plane tabling. PRINCIPLE OF PLANE TABLE SRVEYING- For quick and approximate surveying, when great precision and accuracy is not needed, plane table surveying techniques is very suitable. It is particularly convenient for filling the details between the stations already fixed and surveyed by more precise method of triangulation or 6

63 theodolite traversing. For small area surveys, plane table is recommended. The great advantage of this technique is that field work and map plotting is achieved simultaneously by use of graphical surveying. The principle used in plane table surveying is that an unknown point of interest can be established by measuring its directions from known points. the main principle of plain table surveying is parallelism that the ray drawn from station to the object on the ground is parallel to the line drawn from the station to the object on the drawing sheet. Advantages and Disadvantages Advantages (a) Plane table survey is most suitable for preparing small-scale maps. It is most rapid. (b) The field book is not necessary as plotting is done in field concurrently with the field work, and hence the mistakes in booking the field notes are avoided. (c) The surveyor can compare the plotted work with the actual features of the area surveyed and, thus, cannot overlook any essential features. (d) There is no possibility of omitting the necessary measurements as the map is plotted in the field. (e) Errors of measurements and plotting may be readily detected by check lines. (f) Contours and irregular objects may be represented accurately, since the tract is in view. (g) It is particularly advantageous in magnetic area where compass survey is not reliable. (h) It is less costly than a theodolite survey. (i) No great skill is required to prepare a satisfactory map. Disadvantages (a) The plane table is essentially a tropical instrument. It is not suitable for work in a wet climate. (b) It is heavy, cumbersome, and awkward to carry. (c) There are several accessories to be carried and, therefore, they are likely to be lost. (d) It is not intended for accurate work. 63

64 (e) If the survey is to be re-plotted to a different scale or quantities are to be computed, it is of great inconvenience in absence of the field notes. Equipment The plane table essentially consists of a simple drawing board mounted on a tripod similar to a compass or a level. The drawing board usually made from well seasoned teak or pine wood. The size can vary from mm to mm. Sometimes square boards of mm or mm are also used but size of square boards is rather uncommon. Another important constituent of plane table is a straight edge called Alidade. It is made of a metal (brass or gunmetal) or seasoned wood about 500 mm long with a straight ruled edge which is bevelled. This edge is termed fiducial edge. It may be provided with sight vanes, at both ends in a plain alidade or (Figure 5.(a)) with a telescope for better accuracy as shown in Figures 5.. In plain alidade one of the sight vanes is provided with a narrow slit and the other is provided with cross and stadia wires. Like a level, two bubble tubes placed orthogonally are provided for keeping the plane table horizontal. The beveled edge is graduated so that it can be used as a scale for plotting distances directly on the map. 64

65 Accessories The additional equipment to be used for surveying with plane table could be as given below : Trough Compass It is usually 5 cm long, shown in Figure 5.(a), and is provided to plot the magnetic meridian (N-S direction) to facilitate orientation of the plane table in the magnetic meridian. Spirit Level Circular spirit level is used to check the level of the board and make it horizontal by placing it on the board in two positions mutually at right angles and centering the bubble in each position. Plumbing Fork It is also known as U frame. It is a hairpin shaped brass frame having two arms of equal length as depicted in Figure 5.(b). One end of the frame is pointed and is kept over the drawing sheet touching the plotted position of the instrument station. The other end of the frame carries a plumb bob. The position of the plane table is adjusted until the plumb bob hangs over the station occupied by the instrument. 65

66 Drawing Sheet Drawing paper should be of best quality and well seasoned to minimize the effect of climatic variations. The paper should be tinted green or grey for reducing glaring in sun and eye strains. Drawing paper is fixed on board with drawing pins, clamps etc. For drawing rays and other detail quality pencils, dustless rubber and precision scales are used. A water-proof cover is also an essential accessories to protect drawing paper from dampness and rain. 66

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68 FIELD PROCEDURE Fixing of tripoid stand on the fairly level ground. Fixing of plane table on tripoid stand. Levelling. Centering. Marking N-S line on right hand top corner of plane table. Orientaion. Plotting. METHODS OF PLANE TABLE SURVEYING Radiation Intersection Traversing Resection 68

69 Radiation This is most direct and simple method of recording observations during plane table surveying. The instrument station O is selected and instrument is set and oriented at this station. The point of interest, representing important ground features, natural or created, is located on map plan by drawing a ray from the plane table station to that point with the help of alidade and plotting to scale the measured distance as shown. Small land areas can be surveyed from a single instrument station on one table setting at a predetermined and located position. The instrument station is selected such that entire area is visible and approachable from this position for distance measuring and sighting. The instrument station designated O is plotted on drawing sheet exactly oriented and levelled at ground station O with the help of U frame as depicted in Figure 5.(b). The various survey target points A, B, C etc. are sighted by centering the alidade on O and rays drawn along its edge. The distances O A, O B etc. can be measured by chain/tape and plotted as O a, O b, on the sheet. The N-line is marked at top of sheet with the help of compass. This way the traverse abcdef can be plotted. Accuracy can be checked by measuring ground distances AB, BC etc. and comparing with map distances ab, bc etc. Intersection In place of one ground station O, as in radial method, two ground stations O and O are selected on ground, such that all important features of area to be surveyed are sightable from both stations. The line joining instrument station O and O is termed base line. It is the only distance which is required to be measured linearly on ground. With plane table positioned at one station (say O ) the point is transferred on sheet as O as in last method. With alidade pivoted at O different survey points A, B, C are sighted and radial lines O a, O b, O c are drawn. Next the plane table is shifted and positioned at O. With alidade pivoted at O, survey points A, B, C are sighted again and radial line O A, O B, O C are drawn on sheet. The intersection of radial 69

70 lines, e.g. (O a and O a) will give the location of A on sheet as a and so on, without making any linear measurement as shown. This method is generally preferred for plotting the details of ground, objects, which are far away or difficult to access, rivers etc., and the survey stations which can be subsequently used as instrument stations. It is particularly useful in rough and uneven regions where accurate linear measurements are tedious, or difficult or even impossible in some cases. Traversing The method of radials or intersection (from a base line O O ) can be used preferably for small level surveys. However, plane table can also be used for traversing surveys of wide and large areas similar to chain and compass surveys, for both closed and open traversing. Survey lines O O O can be run between stations which are already predecided by other 3 methods. The topographical details are fixed by plane table traversing. The step-by-step procedure can be described as follows : (a) Traverse stations O, O..., O are predecided on ground. 3 (b) Set and level the table at O and mark o on sheet exactly above O using U frame. Centering the alidade at O, other traverse stations O, O... etc. which can be 3 sighted from O, are observed and rays O O, O O, O O,..., etc. are drawn. 3 4 For topographical details stations A, B, C,..., etc. are sighted and rays drawn as per Figure below. 70

71 (c) The table is then shifted to next station O, fixed levelled and oriented. Position of station O is marked on sheet. Radial rays O O, O O, O O are then drawn with 3 4 alidade centered on O. The intersection of rays O O and O O will give the 3 3 location of station O on sheet and so on. 3 The ground features A, B, C,... etc. can be similarly located on map by drawing rays from station O. Details can also be located by method of radials. (d) The process is continued till completion of survey. (e) Accuracy is checked by sighting station O, O... etc. from more than two stations so that three radial lines merge at referred station. However, if a particular traverse point is not observable from more than two traverse stations, some well defined object on area can be temporarily chosen as instrument station for checking. RESECTION It is a method of orientation employed when the table occupies a position which is not yet located on the drawing sheet. Position of instrument station occupied by the plane table can be drawn on sheet (or map) with the help of two or more well defined points which are visible from instrument station and whose positions have already been drawn on plan map. Simple Problem (Back Ray Method) 7

72 This method is very useful when one of the plotted stations in accessible from the station to be plotted. The procedure of resection after orientation by back ray is given below : (a) Base line O O is selected on ground as distance between two well defined points O and O on ground whose positions are measured and plotted accurately on the plan map. (b) Set, level and orient table at O. Alidade is placed along O O such that signal at O is bisected. With alidade at O another station O is sighted, which is required to be 3 located, draw a line O O, station O is marked on map along this ray approximately. 3 3 (c) Shift the table and set it afresh at O and orient it by backsight on O. 3 (d) Place alidade at O on map, sight O on ground and draw the ray O O. The point of 3 intersection of rays O O and O O will give or locate the position of O on map This process is repeated to obtain positions of all instrument stations O, O... etc. on map. 4 5 Two-point Problem The back ray method requires drawing the ray from preceding stations (O and O ) to the station to be occupied by plane table (say O ). Errors of centering thus are inevitable. 3 The two-point problem consists of locating the position of a plane table station on the drawing sheet by observation of two well defined points, whose positions have already been plotted on plan. The procedure of resection after orientation by two points is given below. (a) Let O O be the two stations plotted as o and o on the drawing sheet. It is required to plot station O for plane tabling work. 3 (b) An auxiliary point A on ground is selected such that AO is approximately parallel to O 3 O and the angle O O A and O O A are balanced angles, i.e. these are neither too 3 3 acute or too obtuse. The table is set and levelled at A, and so oriented that line O O on ground is nearly parallel to line o o plotted on table map. 7

73 (c) Alidade, touching o and sighting O on ground, a ray is drawn through o. In the same way, draw a ray by touching alidade to o and sighting O on ground. This ray will intersect the first ray at a on the map. (d) With alidade touching a, sight O and draw the ray a o. Mark the estimated position 3 3 of O on the map as o. 3 3 (e) The table is removed from A and set at O with marked position of o over O, properly levelled and similarly oriented. This is achieved by back sighting A from O. 3 (f) Now with table at O, keep alidade touching o and sight O and draw a back ray resecting the 3 line a o in o. Here o is the point representing the station O with reference to the approximate orientation made at A. (g) With alidade touching o, sight O and draw a ray to O. If the ray passes through the 3 plotted point o, the orientation of the table is correct and o is the correct position of 3 O. Whereas, if this ray cuts the previously plotted line a o at some other point, say 3 o, then the position o is not the correct position of O

74 (h) The orientation error will be equal to ooo between the lines o o and o o. This error can be eliminated by rotating the table through the angle o o o. This table rotation can be achieved by taking the following steps. (i) The alidade is placed along line o o and a ranging rod B is fixed in line with o o, far away from the plane table. (ii) Alidade is now kept along true line o o and table is rotated so that ranging rod B is bisected. The table is clamped in new position. (iii) The true location of O on map is now marked by : 3 (a) orienting alidade along o O and drawing the ray o O, and (b) orienting alidade along o O and drawing the ray o O. The point of intersection of the two rays will give the correct position of O 3 (the new table position) on map. The new position of table station O is, thus, correctly marked on map with the help of two 3 previous table stations O and O already marked on map. The procedure followed is termed two-point problem in plane table survey. Three-point Problem The position of new plane table station on the map can be correctly located with the help of three well defined points on ground whose positions are already plotted on map. Such a procedure is called three-point problem. It is obvious that locating the position of table by this process is more accurate. However, it is more involved and complex. Let there are three ground stations A, B and C whose positions are marked as a, b and c on the plan map and let these stations are visible from new table station O. It is required to plot the position of O on map as o. This can be achieved by any of the following methods : (a) Mechanical (b) Graphical (c) Trial and Error Mechanical or Tracing Paper Method The process of mechanical method is applied using a tracing paper or cloth. The table is stationed, set and levelled at station O and is oriented as nearly as possible in its correct position either by visual judgment or by use of compass. A tracing cloth/paper is spread and stretched over the table. The position of O is guesstimated and fixed on the tracing to approximately locate the table station O on the map as o. With alidade centered at o, stations A, B and C are bisected 74

75 and rays oa, ob and oc are drawn on the tracing. The tracing is then un-stretched and rotated until the three new drawn rays pass through plotted positions of a, b and c on the map. This will provide a new position of station O on map as o. This is transferred to map by a pin of a fine needle point. The alidade is then placed along o a and station A is bisected by rotating the table and then clamping it in new position. Stations B and C are then sighted and rays drawn as check. The new rays shall pass through o if new table orientation is correct. However, a small triangle of error may be formed as table orientation was only approximate. The above process is then repeated by trial and error till the triangle of error vanishes. Graphical Method Several graphical methods are suggested to solve the three-point problem. However, the Bessel s solution is the most commonly used method in practice being the simplest. The Bessel s solution can be described in the following steps : (a) The plane table is set up and levelled at new station O. The alidade is placed along known line (say ba on the map) and table is rotated until A is sighted with a pointing towards A as shown in Figure 5.8(a), clamp the table and sight C with alidade centered on b, draw a line x-x along alidade edge. (b) The alidade is now placed along ab and table turned to bisect B with b towards B as in Figure 5.8(b). Clamp the table and centre the alidade at a, bisect C by drawing the ray ac intersecting the previously drawn ray x-x at point c (say). Join cc. (c) Alidade is now placed along c c as in Figure 5.8(c) and table turned till C is bisected and clamped in new position. The table is correctly oriented. 75

76 (d) The alidade is centered at b and B is bisected. Draw the ray to intersect cc in o. Similarly, if alidade is pivoted about a and A is sighted, the ray will pass through o if the process is accurate. Any minor error is corrected accordingly. Trial and Error Method or Lehmann s Method This method is very commonly used in field and is quite quick and very accurate. The plane table is stationed, set and levelled at station O and is oriented as nearly as possible into correct position either by visual judgment or by use of compass. Rays Aa, Bb and Cc through plotted points a, b and c are drawn sighting stations A, B and C along aa, bb and cc respectively. If the table was oriented correctly to start with, all these rays 76

77 will intersect at common point o on the map indicating correct position of station O. However, since the initial orientation was only approximate, a small triangle o o o will be formed in place 3 of a common point o. This triangle is called triangle of error and is shown in Figure 5.9(b). This triangle is attempted to shrink to a point by trial and error, so that in final positions lines aa, bb and cc pass through a single point o. The process applied to achieve this object is known as Lehmann s rule. The triangle formed by joining stations A, B and C is termed great triangle while the circle passing through A, B and C as great circle (Figure 5.9a). 77

78 Lehmann s Rule The Lehmann s rule can be stated as follow : (a) The distance of true position of o from each of ray aa, bb and cc is proportional to the distance of O from ground stations A, B and C respectively. (b) If we look in the directions of stations A, B or C, the true position of station O is on the same side of the three rays aa, bb or cc, i.e. if the table station O is outside the great triangle ABC, the triangle of error will be outside the triangle abc and o will be outside of abc. Similarly, if table station O is within the triangle ABC, the triangle of error will be inside abc and o will be inside the triangle of error. (c) If the table station O is outside the great triangle but inside the great circle, the ray to middle station B, bb in Figure 5.9(d) lies between the true station position o and intersection of other two rays (i.e. aa and cc). (d) When table station is outside the great circle, the table position O in Figure 5.9(e) is on the same side of ray towards most distant point (aa) as the intersection of other two rays, e. Using above rules, the triangle of error is sought to be shrunk to a point quickly. The first triangle of error is used to locate new trial position of O (say o ) and placing alidade along o and the one of the known point (say a) and then rotating the table so that A is sighted. 78

79 Clamping the table in new position, B and C are sighted and rays drawn. The new triangle of error is generated which is much smaller than the first triangle of error. New position of table station (say o ) is marked using Lehmann s rules. The process is repeated until all the rays aa, bb and cc intersect at single point o. ERRORS IN PLANE TABLING The main sources of errors in a plane table survey can be broadly classified as follows: (a) Due to faulty instrument adjustments (b) Due to quality of drawing paper used in map plotting (c) Human errors of surveyor in centering and orienting the table (d) Surveyor s error in observing and plotting. Faulty Instrument Adjustments The instrument, if not properly adjusted, will introduce many errors in plane table survey. These adjustments, which are normally required, their methods of testing and subsequently correcting are described in detail in Section 5.4. Quality of Drawing Paper The drawing paper stretched on the plane table board for recording the survey details and plotting the plan shall be of good quality. The expansion, contraction and shrinking of paper due to temperature and moisture changes can cause errors in survey map reading even if it is prepared error free. The errors due to these can be minimized by using the plotted scale. Surveyor s Errors in Table Setting There can be primarily two types of errors which are: (a) Inaccurate centering of table, and (b) Inaccurate orientation of table. 79

80 Surveyor s Error in Observing and Plotting Human error can be introduced during observation and plotting of details by the surveyor. These could be due to objects not being sighted and bisected in sight vanes accurately, The centering of alidade on the desired station point on paper may not be accurate, The radiating ray towards the desired object may not be correctly drawn through the referred station point, and plotting of details may not be properly done or recorded. Care should be exercised during observation process to eliminate these types of errors. Random rechecking of some details recorded at referred instrument station is desirable. 80

81 MODULE- SURVEYING-I SYLLABUS Levelling: Use of dumpy level and leveling staff. Temporary and Permanent adjustment of dumpy level, Reduction of levels by height of instrument and rise and fall method. Curvature and refraction error, sensitiveness of level tube, reciprocal levelling, leveling difficulties and common errors, Automatic and Electronic or Digital levels 8

82 LEVELLING Levelling may be defined as the art of determining the relative heights or elevations of points or objects on the surface of the earth. Therefore, it deals with measurements in vertical plane. Levelling has wide applications in the field of agriculture. Construction of irrigation and drainage channels, terraces, bunds, reservoirs, outlet structures, etc, require the knowledge of surveying. For any soil conservation and land levelling work, levelling is the first job to be taken up. Objectives After studying this unit, you should be able to understand the basic principles of levelling and various terms used in leveling, explain the use and working of different types of levels and other instruments used in levelling, explain various operations and procedures performed during levelling exercise, and describe various methods of contouring and uses of contour maps. Levelling instruments Two instruments are required to determine the reduced levels of points. They are: (i) a level and (ii) a levelling staff. The level is used to provide a horizontal line of sight and the levelling staff which is a graduated rod is used to read the vertical height of the line of sight above the selected station. The level Various types of levels are used for surveying viz.(i) Hand level, (ii) Farm level, (iii) Wey level, (iv) Tilting level and (v) Dumpy level etc. The dumpy level is widely used for levelling works. For small and rough levelling works, the hand levels and farm levels are used. The dumpy level is very sturdy, compact and stable equipment. The telescope is rigidly fixed to the frame. Therefore, the telescope cannot be rotated about the longitudinal axis and also cannot be removed from the support. Because of its simple features and versatile usefulness, it is widely used. 8

83 Dumpy level The dumpy level is simple, compact and stable. Main parts of a dumpy level are shown in fig.0. A levelling instrument essentially consists of tripod or three legged stand, levelling head mounted on the tripod, the limb, telescope and the bubble tube. The most important part is the telescope which may be either internal focusing or external focusing type. A levelling head is mounted on the tripod stand having two parallel plates and three or four foot screws. The limb, consists of the vertical axis and a horizontal plate, connects the levelling head with the above telescope. Dumpy level The telescope has an object glass at the forward end and eye piece at the rear end. The eye piece magnifies the image of the object formed by the object glass. All the parts above the levelling head are capable of rotating round the vertical axis. One or two bubble tubes are provided for leveling the instrument. The bubbles can be brought to the centre of the bubble tubes by adjusting the foot screws, which support the upper parallel plate. The diaphragm is fixed a little beyond the eye-piece inside the main tube. The diaphragm houses a brass ring which is fitted with cross hairs. There are three sets of horizontal hairs. The central cross hair gives the line of sight. The line joining the intersection of the central cross hair to the optical centre of the object glass and its continuation is the line of sight. When sighted through the eye piece, continuation of the above line meets the leveling staff at a point denotes the staff reading. 83

84 The levelling staff There are various types of graduated staves. Out of all, the "sop with telescopic staff" is commonly used. The purpose of a levelling staff is to determine the amount by which the station (foot of the staff) is above or below the line of sight. Sop with telescopic staff It is a straight, rectangular, graduated rod with the foot of the staff representing the zero reading. The width and the thickness of the staff are 75 mm and 8 mm respectively. It is made of well seasoned wood such as cypress, blue pine or deodar free from any defect. It is arranged in three telescopic lengths. It is usually 5 m long when fully extended (Fig.). The solid top length of.5m slides into the central box of.5 m, which in turn slides into lower or bottom box of.0 m length. Each length when pulled out to its full length is held in position by means of a brass spring catch. Each metre is subdivided into 00 divisions, the thickness of graduation being 5mm. Spaces indicating the decimetre readings are marked in red while all other spaces are marked in black against a white background. Each decimetre length is figured with the corresponding numerals, the metre numeral is in red and marked to the right and the decimetre numeral in black and marked to the left. When viewed through the telescope, the staff appears inverted and therefore, readings are taken from top to downwards. Terminology connected with leveling Datum: It is also called datum plane or only datum. A datum surface is usually an imaginary level surface or arbitrarily assumed level surface, from which vertical distances are measured. Its elevation is zero. In India, the datum adopted for the Great Trignometrical survey (GTS) bench mark is the mean sea level at Karachi, now in Pakistan. At present, the mean sea level at Madras is used. Level Surface Any level surface parallel to mean spherical surface of earth is called a level surface. This surface is normal to the direction of gravity (indicated by plumb bob). Every point on this surface is equidistant from centre of earth. A plane tangential to level surface is called the horizontal plane at that point. Any line lying in the horizontal plane is a horizontal line as shown in Figure 84

85 Vertical Plane The plane normal to horizontal plane at any point will be the vertical plane. This plane will contain the plumb line drawn through that point. The angle of intersection between two lines in a vertical plane is called vertical angle. It is normal to select horizontal line as one of these two lines to measure the vertical angle Datum Since the actual ground surface of earth is undulating, one reference line has to be decided to obtain the relative heights of points on ground on the surface of earth in the plot of area surveyed. This arbitrarily decided level surface is called datum surface. The heights of different points in surveyed area are measured with reference to this level surface. In India the datum was fixed as mean sea level at Karachi during Great Triangulation Survey (GTS). This datum is still being used for benchmarking in all precision surveys. Elevation: It is the vertical distance above or below the datum. It is also known as reduced level (R.L.) The elevation of a point is plus or minus according as the point is above or below the datum. Bench Mark (B.M.) : It is a fixed point of reference of known or assumed elevation with respect to which other elevations are calculated. It is a starting point for leveling. Temporary bench marks are selected at the end of a day s work. There are four kinds of Bench marks. (a) G.T.S (Great Trigonometrical Survey) Bench Mark: These bench marks are established with very high precision at intervals all over the country by Survey of India department. Their position and elevation above the standard datum are given in the catalogue published by the department. (b) Permanent Bench Mark: These are the fixed points of reference established between the GTS 85

86 bench marks by Government agencies such as PWD. On clearly defined and permanent points such as top of the parapet wall of a bridge or culvert, corner of a plinth of a building, gate pillars etc., (c) Arbitrary bench Marks: These are the reference points whose elevations are arbitrarily assumed. They are used in small levelling operations. (d) Temporary Bench Marks: These are the reference points established at the end of day s work or when there is a break in the work. The work, when resumed, is continued with reference to these bench marks. Line of collimation: It is the line joining the intersection of the cross hairs to the optical centre of the object glass and its continuation. It is called the line of sight. Axis of telescope: It is a line joining the optical centre of the object glass to the center of the eye piece. Axis of the level tube or bubble tube: It is an imaginary line tangential to the longitudinal curve of the tube at its middle point. It is also known as bubble line. It is horizontal, when the bubble is centered. Height of the instrument: It is the reduced level (R.L.) of the plane of sight when the leveling instrument is correctly leveled. It is also called the "height of the plane of the collimation" or the collimation. The line of collimation will revolve in a horizontal plane known as plane of collimation or the plane of sight. Back sight: It is a staff reading taken on a point of known elevation, as on a bench mark or a change point. It is also called a plus sight. It is the first staff reading taken after the level is set up and levelled. Foresight: It is the last staff reading denoting the shifting of the level. It is the staff reading taken on a point whose elevation is to be determined. It is also termed as a minus sight. It is the last staff reading, denoting the shifting of the instrument. Change point: It is the point on which reading is taken just before and after shifting the instrument. That means both back sight and fore sight readings are taken on this point. It is also called a turning point. It should be taken on a firm, well-defined object. 86

87 Station: A station is a point whose elevation is to be determined or a point which is to be established at a given elevation. Leveling procedure - temporary adjustments in dumpy level, level field note book, recording procedure in level field note book. Taking out the instrument from the box Before taking out the instrument from the box, mark the positions of (a) the object glass, (b) eyepiece (c) clamp and tangent screws so that while keeping back it can be placed in the box in proper position without any difficulty. 9. Adjustment of the level The adjustments of a level are of two kinds: (a) Temporary adjustments And (b) Permanent adjustments 9.. Temporary Adjustments: The temporary adjustments are those, which have to be done at each set-up of the level. They are necessary adjustments to take readings. They are: (a). Setting up the level, which include (i) Positioning the tripod, and (ii) Levelling up (b). Focussing the eye-piece and object glass to eliminate parallax. 9.. Setting up the level 9... Fixing the instrument on the tripod: Release the clamp screw of the instrument, hold the instrument in the right hand and fix it on the tripod by turning round only the lower part with the left hand. Screw the instrument firmly Leg adjustment: Plant the instrument at the desired point at a convenient height for sighting. Spread the tripod legs well apart and tri-brach sprang as nearly level as can be judged by the eye. Bring all foot screws in the centre of their run. Fix any two legs firmly into the ground by pressing them with the hand and move the third leg to the right or left until the main bubble is approximately in the centre. Then move it in or out until the bubble of the cross level is approximately in the centre. It is only approximate levelling Levelling up: Place the telescope parallel to a pair of foot screws and bring the bubble to the centre of its run by turning these screws equally either both in wards or both outwards. Turn the telescope to 900 so that it lies over the third foot screw and centre the bubble by turning this screw. Repeat the operations until the bubble remains in the centre of its run in both positions. Once this operation is complete, the bubble should remain in the centre for all directions of the telescope, provided the instrument be in correct permanent adjustment. 87

88 9...4 Focussing the eye piece: Remove the lid from the object glass and hold a white paper in front of it. Move the eye piece in and out until the cross hairs on the diaphragm are seen distinctly Focussing the object glass: Direct the telescope towards the staff and on looking through the eye-piece, bring the image of the staff between two vertical hairs of the diaphragm by lightly tapping the telescope. Adjust the objective by turning the focusing screw until the parallax error is eliminated. Focussing: Adjusting of the eye-piece and the objective at the proper distance apart for the clear vision of the object sighted is known as focusing. First, focus the eye-piece by holding a white paper in front of the telescope and move the eye- piece in and out until the cross hairs appear distinct and clear. Then focus the object glass by directing the telescope towards the object and turn the focusing screw until the image appears clear and sharp. By focusing, the focus of the objective and that of the eye-piece coincide with cross hair of the diaphragm as the diaphragm is placed at the common focus. Parallax: The apparent movement of the image relatively to the crosshairs when the image formed by the objective does not fall in the plane of the diaphragm is called "parallax" and the process of precise focusing on the staff is often called "adjusting for parallax". If the image appears to move in the same direction as that of eye, it is in front of the diaphragm and the focusing screw must therefore move the objective inwards. If however, the image appears to move in the direction opposite to that of the eye, it is beyond the diaphragm towards the eye piece and the objective therefore to be moved outwards by the focusing screw. It may be noted that parallax error can be eliminated wholly by slightly turning the focusing screw backwards or forwards until such motion no longer exists. Holding the staff: While reading the staff, care should be taken in holding the staff in vertical. The staff man stands behind the staff, heels together, with the heel of the staff between his toes and holds it between the palms of his hands at the height of his face in a vertical position. The readings will be too high, if the staff is not vertical. Special care must be taken with the larger readings, since the errors due to a given deviation from the vertical vary with the readings. The staff should be very slowly waved forward towards the level and backwards away from it. The person reading the staff should record the lowest reading which will be the correct reading. 88

89 9.3 Reading the staff: Direct the telescope towards the staff held vertically on the station after the instrument is leveled. Bring the staff between the two vertical hairs and use the portion of the horizontal cross hair between them in reading the staff. Bubble should be in the centre of its run while reading the staff. 9.4 Booking staff readings in level book: The readings should be entered in the respective columns and in the order of their observation. The first reading is obviously a back sight and should be entered in that column. A remark should be made in the remarks column describing weather back sight (B.S.) is taken on a permanent or temporary or arbitrary bench mark (B.M.) and its value should be noted. If more than one reading is taken from the same position of the instrument, all the subsequent readings should be recorded in the intermediate sight column. The last reading is a change point (C.P.) and recorded in foresight column (F.S.). The foresight and back sight of the change point should be written in the same horizontal line. The R.L of the plane of collimation should be written in the same horizontal line opposite to B.S. If the last entry at the bottom of the page happens to be an intermediate sight (I.S.), it should be repeated as the first entry on the next page and should be recorded both in I.S and F.S columns. The arithmetic checks should be made and written at the bottom of every page at home, the same day the levelling is done, so that if any discrepancy is found, it can be checked the next morning in the field. Permanent Adjustments The three critical axes of a dumpy level are line of collimation, bubble tube axis and instruments vertical axis as shown in Figure 4.5. These are correlated in following way with each other. (a) Line of collimation and bubble tube axis are parallel to each other. (b) The bubble tube axis is normal to vertical axis of the instrument. 89

90 These conditions are ensured by the manufacturer during production of instrument. However, due to continuous usage some wear and tear do occur and the above relationships between critical axes are disturbed. This will introduce instrumental error in level measurements. For accuracy, the level is required to be adjusted so as to satisfy the above conditions. Since these adjustments are needed only when the instrument s internal setting is disturbed during usage and not required every time the instrument is setup, these are termed permanent adjustments. When the instrument is not adjusted, the line of collimation will not be parallel to the bubble tube axis (horizontal) but inclined either upwards or downwards (Figure 4.6). Let this inclination be α. An error is introduced in recording the levels. Let us determine the true difference in levels of station A and B situated at a distance of d and d respectively A B from instrument setup at O. Then line aob represents the true line of collimation (horizontal). However, since the adjustments are out of order, the actual line of collimation will be aob with downward dip and aob with upward dip of line of collimation. 90

91 If Aa = observed reading of staff at station A Then error in reading at A will be Aa Aa = d tan α A Similarly, when staff is held at station B, Bb = Observed reading; Bb = actual reading Error in reading at B is Bb Bb = d B tan α. True difference in levels of stations A and B will be Bb Aa = {Bb d = {Bb Aa } {d B tan α} {Aa B tan α d A d tan α} A tan α}... (4.a) Eq. (4.a) will indicate the error in adjustment and also indicate that it is proportional to distance of staff station from instrument station. One way to eliminate this error would be to keep the instrument station O at exactly equidistant from staff stations A and B, i.e. d = A d B. It will modify Eq. (4.a) as [Bb Aa] = True level difference at A and B = [Bb Aa ] Reduction of levels - height of collimation method, rise and fall method, and numerical problems connected with these two methods. Determination of reduced level Whenever any leveling is to be carried out, the first reading is taken on a point of known elevation. This is called back sight (B.S.) reading. Before shifting the instrument one reading is taken on a firm object whose elevation is to be determined. This is known as fore sight (F.S.) reading. Between the B.S and F.S numbers of readings known as intermediate sights (I.S) are taken. All these readings are required to be tabulated and converted to reduced levels (R.L) for practical use. There are two systems of working out the reduced levels of points from the staff readings in the field: (i) the collimation or the height of instrument (H.I.) and () the rise and fall system. 9

92 0. The collimation system: At first, the R.L. of the plane of collimation i.e., height of instrument (H.I) is calculated for every setting of the instrument and then R.L. of different stations re calculated with reference to the height of the instrument. In the first setting, the H.I. is calculated by adding the B.S. reading with the R.L. of the bench mark. By subtracting all the readings of all the intermediate sights and that of the first change point from the H.I., then their reduced levels are calculated. The new H.I is calculated by adding the B.S. reading with the R.L. of the first change point. The process is repeated till the entire area is covered. Arithmetical check: The difference between the sum of back sights and the sum of fore sights should be equal to the difference of first and last R.L. 0. Rise and fall system: The level readings taken on different stations are compared with the readings taken from the intermediate proceeding stations. The difference in the readings indicates rise or fall depending upon whether the staff reading is smaller or greater than that of the preceding reading. The rise is added and fall is subtracted from the R.L. of a station to obtain the R.L. of the next station. Arithmetical check: The difference between the sum of back sights and the sum of fore sights is equal to the difference between the sum of the rise and fall and should be equal to the difference of first and last R.L. If the R.L. of A is known, the R.L. of B may be found by the following relation: R.L of B = R.L. of A + B.S. - F.S. The R.L s of the intermediate points may be found by the following relation: R.L. of a point = R.L of B.M + B.S. - I.S The difference of level between A and B is equal to the algebraic sum of these differences or equals the difference between the sum of back sights and the sum of the foresights (B.S - F.S.). If the difference is positive, it indicates that the point B is higher than the point A, while if the negative, the point B is lower than the point A. Example: The following consecutive readings were taken with a dumpy level: 0.565, 0.854, 0.940,.005, 0.640, 0.660, 0.785, 0.800, 0.635,.35, and.40 The level was shifted after the fourth and the seventh readings. The first reading was taken on the bench mark of R.L. is Calculate the reduced levels of the change points, and the difference of level between 9

93 the first and last points. Solution: The level was shifted after the fourth and the seventh readings. Hence, the fourth (.005) and seventh readings (0.785) are fore sight readings. Where as the fifth (0.640) and eight reading (0.800) are back sight readings. The starting reading is back sight reading. The last reading is fore sight reading. B.S F.S B.S F.S B.S 0.565, 0.854, 0.940,.005, 0.640, 0.660, 0.785, 0.800, 0.635,.35, and.40 F.S (ii) Collimation or Height of instrument (H.I.) method Arithmetical check: The difference between the sum of back sights and the sum of fore sights should be equal to the difference of first and last R.L. Sum of B.S =.365;sum of F.S =3.0; Ist R.L. =00.000; Last R.L.= = = -.05 (Checked) (i) Rise and fall method 93

94 Arithmetical check: The difference between the sum of back sights and the sum of fore sights is equal to the difference between the sum of the rise and fall and should be equal to the difference of first and last R.L.sum of B.S =.365;sum of F.S =3.0; Ist R.L. =00.000; Last R.L.=98.795; sum of rise = 0.65;sum of fall = = = = -.05 = -.05 (Checked) Types of leveling - simple, leveling, differential leveling and profile leveling. Types of Levelling. Simple leveling: It is the simplest operation in leveling when it is required to find the difference in elevation between two points, both of which are visible from a single position of the level. If the two points are so close that they can be seen from a single set up, their level difference can be determined easily. 94

95 Procedure: If the two points are so close that they can be seen from a single set up, their level difference can be determined easily. Let A and B be two points (Fig.) located closely and it is desired to know their elevation difference. The level can be set up anywhere from where both the stations are visible i.e., at O. But to eliminate the effect of any instrumental error, it is advisable to place the instrument at equal distance from both the stations, but not necessarily in the same line. Staff readings are taken on both the stations. The difference in reading gives the elevation difference between the points. Simple leveling Differential levelling: It is the method of levelling to determine the elevation of points located at some distance apart or to determine the elevation difference between two points or to establish bench marks. The method is used in order to find the difference in elevations between two points: (i) if they are far apart, (ii) the difference in elevation between two points is too great and (iii) if there are obstacles intervening. The method of simple leveling is employed in each of the successive stages. The process is also known as compound or continuous leveling. Suppose, it is required to find the difference of level between two points A and B, which are too far apart as shown in fig. Procedure When two points are located at a distance so that they cannot be viewed from a single set up of the level, then it is required to take a number of change points. Let A and B are two such points (Fig. 3) whose elevation is to be found out. First, the instrument is set up between A and B and the instrument is leveled and focused. Reduced level of A is assumed and is taken as bench mark. From the same set-up, the staff reading (B.S.) at A is taken. The instrument remains in its position and staff is shifted towards B and fore sight (F.S.) reading is taken on this point, which is the first change point. The distance of the change point from the level should not exceed 00m. The level is shifted towards B and set up at a convenient point to keep its distance from the first change point approximately same as before. The process of taking B.S and F.S. reading is repeated till the point B is reached. Enter last station reading in the fore sight. 95

96 The readings are tabulated and the reduced levels of all stations can conveniently be calculated following the collimation or rise or fall system. Differential leveling Profile leveling: It is the method of determining the level of ground surface along a predetermined line which may be the centre line of a road, canal, railways or pipeline. The predetermined line may be a single straight line or a series of connected straight lines. The method is also known as longitudinal leveling or sectioning. Sectioning is useful for laying out roads, canals, terrace lines, contour bunds etc. Procedure for profile leveling: The leveling operation should start from a bench mark. If no bench mark is available nearby, fly levels may be taken to establish temporary bench marks. Depending upon the precision required, the interval at which level should be taken is decided. The fixed interval may be 0, 0, 5 m etc. But apart from this fixed interval, level readings must be taken at all points where there is abrupt change of slopes. If these points are omitted, there will be serious misrepresentation of the nature of slope. The line AH (Fig.4) along which profile leveling will be carried out is located and the points are marked by pegs. Here the R.L. of the bench mark (A) is known and A is also the first point of the line through which profile will be run. The fore bearing of the line should be measured at A by using a prismatic compass. The magnetic compass fitted with the dumpy level may also be used for measuring the bearing of the line. If there are number of connected lines, then the bearings of each line should be measured as the survey progresses. For taking levels, the instrument is set up on a firm ground at M located outside the line AH. Back sight reading is taken on the staff held at A (bench mark). This is a plus sight as this reading added with the R.L. of A gives the H.I. Now the staff is shifted to different points already marked and numbers of I.S. readings are taken. The reading of the last 96

97 clearly visible station (station no.6) is the F.S. reading. This is used as the first change point. After taking the F.S. reading on the first change point, the instrument is shifted to a new position (N) from where maximum number of stations can be covered. The staff-man continues to hold the staff at the same position (C.P. ) till B.S. reading on it is taken. After this, the staff is shifted to different stations and intermediate sights (I.S.) are taken as long as the stations are visible from this set up of the leveling instrument. At last, a change point (C.P.) is selected on a firm ground and the F.S. reading is taken. The instrument is shifted and the process is continued till all the stations are completed. Whenever there is a change in the direction of the profile line, at the point of change, the back bearing of the preceding line and the fore bearing of the succeeding line must be taken. The chainage of all the staff points should be taken continuously from the starting point to the last point. As far as possible the B.S. and F.S. distances should be approximately equal. In addition to the profile readings, staff readings should also be taken on all important features. Also the positions of the features like road, canal, river, fences etc. may be located by taking offsets or by some other means. The level readings should be checked by connecting it with a nearby permanent bench mark. If, no such bench mark is available, the work can be checked by taking fly levels to the original bench mark. Check Levelling It is normal to run a line of levels to return to start station after the end of each days work for the purpose of checking the accuracy and reliability of the measurements and recording carried out on that particular day. This is termed check levelling. This is also carried out to check the particular set of levels fixed previously, or to validate their accuracy. 97

98 FLY LEVELLINGFly leveling is a leveling that is done to connect benchmark to the starting point of the survey line. In this leveling only back sight and fore sight readings are taken and auto level is moved strictly on the line joining benchmark and starting point of survey line. Reciprocal Levelling Let the level difference between stations A and B is required to be measured precisely, and it is not possible to set up instrument midway between A and B (Figure 4.8). At first, instrument is set up and levelled very near to A and staff is held at A and B and readings recorded respectively. The instrument is then shifted to a position O very near to station B, and staff readings once again taken for station A and B respectively. Let a and b be the readings from position O, while a and b are readings with instrument at O. If there is no collimation error, the line of collimation should coincide with horizontal line. If there is no error due to curvature and refraction etc., the level line should merge with horizontal line. Thus, in an error free environment there shall be a single line in place of three, both in instrument position at O (Figure 4.8(a)) and at O (Figure 4.8(b)). Since instrument position O is very near to or exactly at A, there will be practically no deviation in staff reading at A (i.e. reading a is correct) while the true reading (level difference between A and B) would be d = (b e) a... (4.) Similarly for instrument position very near to or exactly at B, the true level difference would be 98

99 d = b a + e... (4.3) Adding Eqs. (4.) and (4.3), we get d = (b a ) + (b a ) or d = / {(b a ) + (b a )}... (4.4) Eq. (4.4) does not have a e term hence is error free. The magnitude of error e is obtained by subtracting Eq. (4.3) from Eq. (4.). i.e. e = / {(b a ) ( b a )} This process of reciprocal levelling eliminates errors due to collimation and curvature. The refraction error may not be completely eliminated as there is a possibility that refraction of the air may change during shifting of instrument from O to O. Hence for more accurate results and to eliminate any possible refraction error, two independent instruments are set at stations O and O simultaneously and readings a, a, b and b recorded at the same instant. SensitivityThe sensitivity is an important specification for a spirit level. The overall accuracy of the device depends on its sensitivity. The sensitivity of a level is defined as the change of angle or gradient required to move the bubble by unit distance. If the bubble housing has graduated divisions then the sensitivity refers to the angle or gradient change required to move the bubble by one of these divisions. Sensitivity of a spirit level is expressed in mm/m. mm is the standard spacing for graduations. The standard sensitivity for a machinst level is.0005 inch per mm division (0 seconds arc). Errors in Levelling The sources of errors in levelling exercise can be several depending upon the location, instrument employed and human resource. The major sources can be listed as follows (a) Instrumental errors. (b) Human errors in setting. (c) Natural causes. 99

100 Instrumental Errors (a) The focusing tube may be faulty causing some tilting in line of sight while focusing. (b) The bubble may be sluggish or insensitive. It can remain in central position even when bubble tube is not horizontal. (c) More common and serious instrumental error is maladjustment of level. The bubble tube line and collimation line do not remain parallel. Even when the bubble tube is horizontal, the collimation line may remain inclined. (d) The staff graduations may not be accurate giving wrong results. Human Errors Inaccurate levelling of instrument by surveyor while setting the instrument, or settling of level during surveying introduces errors. The error is cumulative. The error can be avoided by taking care to set the level in a firm ground and levelling it carefully. If setting on soft ground cannot be avoided, the legs of level tripod are kept on wooden platform or on stakes driven in the ground. The same precaution can be taken at change and intermediate stations to avoid staff settlement. Care should be taken to avoid any contact with tripod while sighting and taking the staff reading. Other human errors could be error in focusing or staff not being held perfectly vertical while taking the level readings, wrong recording of readings or recording in wrong columns etc. Natural Causes These are effects of wind and sun. Considerable difficulty could be experienced while taking the staff reading under glaring sun, or sun shining on the objective glass. Accuracy of observation can also be affected when the velocity of wind is large or when the atmosphere is heated. When the sights are long during precision levelling the errors due to effect of curvature and refraction shall be taken into account. The line of level, defined as a line of equal altitude, will not remain horizontal in long sights due to earth s curvature (Figure 4.). Aa will be the recorded level at A while the real level should be Aa. Thus, an error e = aa is introduced due to earth s curvature given as e = D, where D is the distance in kilometer (km) from the level to the staff c station, and e is in meters. In normal levelling, sight length is less than 300 m, hence e will always be less than m. 00

101 Errors due to refractions are introduced due to refraction of light passing through layers of air of different densities. The bent light ray from staff to instrument will not remain horizontal (Figure 4.3) but will be curved introducing error aa. The effect of refraction is not constant but varies with atmospheric conditions. However, on an average under normal atmospheric conditions the correction for refraction will be aa. The error, e (in meters) = r 0.0 D (i.e. roughly about /7 the correction due to curvature and opposite in sign). The combined correction due to curvature and refraction would be e = e e = ( ) D = D co c r As the effect of curvature is to increase the staff reading so the correction for curvature is subtractive. The correction for refraction is additive to staff reading. Hence, the combined correction is subtractive to staff reading. 0

102 MODULE-III SURVEYING-I SYLLABUS Contouring: Contour interval and horizontal equivalent, characteristics of contours, methods of contouringdifferent and indirect method, contour gradient Theodolite Survey: Use of theodolite, temporary adjustment, measuring horizontal and vertical angles, theodolite traversing 0

103 CONTOURING Contour lines on maps are plotted to show the variation in the elevation of earth surface in plan for various engineering purposes. Contours are used in a variety of engineering works like location of roads, canals, water supply, water distribution, planning and designing of dams, reservoirs, aqueducts, transmission lines, estimating capacity of reservoirs etc. Definitions Contour A contour is an imaginary line on the ground passing through points of equal elevation. It may also be defined as a line in which a level surface intersects the earth s surface. A contour line on a map represents a contour of particular elevation. Contour Interval The vertical distance between any two consecutive contours is called contour interval. Contour interval is kept constant for a contour plant. Contour interval depends upon the following factors : (a) Nature of the ground, (b) Scale of the map, (c) Purpose and extent of survey, and (d) Time and funds. Nature of the Ground Contour interval varies with the topography of the area. If the ground is steep, the contour interval will be large, whereas for flat grounds the contour interval will be small. Scale of the Map Contour interval is inversely proportional to the scale of the map. If the scale is large the contour interval should be small, whereas if the scale is small the contour interval should be large. Purpose and Extent of Survey Purpose and extent of survey affects the choice of contour interval, e.g. small contour interval is used for a survey intended for detailed design work and for accurate earthwork calculations. A large contour interval is used when the extent of survey is large, e.g. location surveys for communication lines, highways and railways. Time and Funds If the time and funds are short and limited the contour interval is kept large. 03

104 Horizontal Equivalent The horizontal distance between any two consecutive contours is known as horizontal equivalent. For a given contour interval horizontal equivalent depends upon the steepness of the ground. Use of contours (i) By inspection of a contour map, information regarding the characters of the terrain is obtained, whether it is flat, undulating or rolling etc. (ii) Contour map is very useful for taking up land leveling works. (iii) With the help of contour map, suitable site for reservoirs, canal, drainage channels, roads, railway etc. can be selected. (iv) Total drainage area and capacity of reservoirs can be determined with the help of contour map. (v) Computation of earth work is possible from contour map. (vi) Contour maps are essential for taking up any soil conservation works like terracing, bunding, construction of structures and spillways. (vii) In coastal areas for construction of brackish water fish farm contour map is required to decide about the type of farm to be constructed i.e. tide-fed or pump-fed farm. (viii) Intervisibility of any two points can be known from the contour map. (ix) From the contour map of agricultural land, most suitable method of irrigation for a particular crop can be decided. 04

105 (x) Section can easily be drawn from contours. (xi) A route with a given slope can be traced on a contour map. Characteristics of contour lines: (i) All points of contour line have the same elevation. (ii) Uniformly spaced contour lines indicate uniform slope whereas, straight, parallel and equally spaced lines indicate a plane surface. (iii) Widely spaced contour lines indicate a flat ground and closely spaced contour lines indicate steep ground. (iv) A series of closed contours with the higher values inside indicate a summit or hill (v) A series of closed contours with the higher values outside indicate a depression (vi) Contour lines cross a ridge or a valley line at right angles. (vii) If the contour lines form U-shaped curves and higher values of contour are inside the loop, then it indicates a ridge line (vii) If the contour lines form V-shaped curves and the lower values of contour are inside the loop, it indicates a valley line (viii) Contour lines cannot cross one another or merge on the map except in case of an over hanging cliff (ix) If several contour lines coincide i.e. the horizontal equivalent is zero then it indicates a vertical cliff (x) Four sets of contours shown in represent a saddle i.e. a depression between summits. It is a dip in a ridge or the junction of two ridges. Line passing through the saddles and summits give watershed line. 05

106 06

107 07

108 Methods of Locating Contours Methods of locating contours may be classified as (a) direct, and (b) indirect. Direct Method In this method, the contour to be plotted is actually located on the ground with the help of a level or hand level by marking various points on the contour. These points are surveyed and plotted to draw the contours through them on the plan. Though the method is slow and tedious but it is most accurate and is used for contouring small areas with great accuracy. In contouring, field work consists of horizontal and vertical control. For a small area, horizontal control can be performed by a chain or tape, while for a large area compass, theodolite or a plane table can be employed. For vertical contour, a level and staff or a hand level may be used. Vertical Control by Level and Staff A series of points having same elevation are located on the ground in this method. An instrument station on the ground is selected so that it commands a view of most of the areas to be surveyed. Height of the instrument can be fixed sighting a nearest benchmark. Staff reading is calculated for a particular contour elevation. The staff man is directed to move left or right along the 08

109 expected contour until the required reading is observed. A series of points having same elevation as shown by the same staff reading are plotted and joined to get a smooth curve. Vertical Control by Hand Level The same principle as used in level and staff method is employed in this method also. This method is very rapid in comparison to the former method. A hand level or an abney may be used to get an indication of the horizontal line from the eye of the observer. A level staff or a pole having zero mark at the height of the observer s eye which is graduated up and down from this point is used in this method. The man with the instrument stands over the benchmark and the staff man is moved to a point on the contour to be plotted. As soon as the man with instrument observes the required staff reading for a particular contour he instructs the staff man to stop and locates the position of the point. Indirect Method Indirect methods are quicker, cheaper and less laborious than direct method. In this method, a series of guide points are selected along a system of straight lines and their elevations are determined. These points are then plotted and contours are drawn by interpolation. The guide points generally are not the points on the contours to be located except in case of a coincidence. For plotting of contours, the interpolation is done with the assumption that the slope between any two adjacent guide points is uniform. Some of the indirect methods of locating ground points are given below. Methods of Squares This method is very suitable when the area to be surveyed is small. This method is also called coordinate method of locating contours. The area to be surveyed is divided into a number of squares forming a grid. The side of a square may vary from 5 to 0 m depending upon the nature of the contour and contour interval. The elevations of the corner of squares are then determined by using a level and a staff. The levels are then interpolated and contour lines are drawn. Sometimes rectangles may also be used in place of squares. 09

110 Method of Squares for Locating Contours Methods of Cross-sections This method is generally used in root surveys. Cross-sections are run transverse to the centre line of a canal, road and railway etc. The spacing of cross-sections basically depend on the nature of terrain and the contour interval. The reduced level of various points along the section line are plotted on the plan and the contours are then drawn by interpolation. Tacheometric Method This method is suitable for hilly areas. In this method, a number of lines are set out radiating at a given angular interval from different traverse stations. The representative points on these lines are located in the field by observing vertical angles and the staff reading of the stadia wires of a tacheometer. The elevations and the distances of these points are calculated and plotted and then contour lines are drawn by interpolation. Interpolation of Contours Interpolation of contours is the process of spacing the contours proportionally between the plotted ground points. Contours may be interpolated by the following methods. By Estimation This is a rough method and is used on a small scale maps. In this method, position of contour points between ground points are estimated and the contour lines are drawn through these points. 0

111 By Arithmetic Calculations This is very accurate method but time consuming also. Position of contours points between guide points are located by arithmetic calculations. For example, A and B are two ground points having their elevations as m, and m respectively. The distance between these points is 5 m and let the contour interval is m. Let between A and B the contours of 54 m, 55 m and 56 m are to be located. The contours can be located as follows : Difference of level between A and B = = 3.0 m. Difference of level between A and the 54 m contour point = = 0.35 m. Hence, distance of 54 m contour point from A = (0.35/3.) 5 =.64 m. Similarly, the distances of 55 m and 56 m contour points from A can be calculated as 6.33 m and.0 m, respectively. These distances can be plotted to the scale on the map. By Graphical Method This method is rapid and convenient when high accuracy is required and interpolation work is too much. In this method, a tracing paper/cloth is used to draw parallel lines at some fixed th intervals, say 0.5 m, at equal intervals and every 0 line is made thicker. Let A and B are two points of elevation 57.5 m and 68.5 m, respectively. Suppose it is required to interpolate 5 m contours between A and B. Assume that bottom or zero line represents an elevation of 55 m and the successive thicker line represents 60 m, 65 m, 70 m etc. Place the tracing paper so that the th th point A is on the 5 line. Now, turn the tracing paper until the point B is on the 7 line from the th st nd zero line (or on the 7 line from the second thicker line). The intersections of and thicker line representing elevations 60 m and 65 m and the line AB will give the positions of the points on the 60 m and 65 m contours, respectively. These positions are pricked through on to the plan. Applications Some of the important uses of contour maps are as follows : (a) Intervisibility of two given points can be ascertained from the contour map. (b) Inspection of contour map can provide information regarding character of the tract of the country, e.g. whether it is flat, undulating or mountainous etc.

112 (c) Contour maps can help in selection of most economical and suitable site for engineering works like reservoir, canal, sewer, road or railway. (d) Earthwork computation can be done with the help of the contour maps. (e) From a given contour plan the section along any given direction can be drawn to know the general shape of the ground or to use it for earthwork calculations. (f) Contour plan may be used to calculate the capacity of the reservoir. Contour Gradient An imaginary line on the surface of the earth having a constant inclination with the horizontal (slope) is called contour gradient. The inclination of a contour gradient is generally given either as rising gradient or falling gradient, and is expressed as ratio of the vertical height to a specified horizontal distance. If the inclination of a contour gradient is in 50, it means that for every 50 m horizontal distance, there is a rise (or fall) of m.

113 THEODOLTE SURVEYING INTRODUCTION The introduction of theodolite as an essential equipment for any exhaustive, accurate and extensive survey exercise like triangulation and precise measurement of horizontal and vertical angles, contouring and even measuring linear distances under difficult terrain conditions has already been covered in the first course on survey. -Theodolite is used to measure the horizontal and vertical angles. -Theodolite is more precise than magnetic compass. -Magnetic compass measures the angle up to as accuracy of 30. However a vernier theodolite -measures the angles up to and accuracy of 0, 0. -There are variety of theodolite vernier, optic, electronic etc. Objectives After studying this unit, you should be able to conceptualize about adjustments, understand various methods of traversing, understand traverse adjustments and measurements, and conceptualize about traverse computations. Uses of Theodolite; Mapping applications and in the construction industry Measurement of Horizontal and vertical angle Measurement of magnetic bearing of lines Locating points on line Prolonging survey lines Determining difference in elevation Setting out curves Aligning tunnels Mining works etc. Three assemblies of Theodolite 3

114 Vernier theodolite is also known and transit. A transit theodolite is one in which the telescope can be rotated in a vertical plane. Main parts of a theodolite Levelling head (7): Levelling head is used to attach the instrument to tripod and attach the plumb bob along the vertical axis of the instrument. 4

115 TRANSIT THEODOLITE Vertical circle Horizontal Axes Plate bubble Levelling Head Vertical axis Vertical circle clamping screw standard Upper plate clamping screw Upper plate Lower plate clamping screw Foot Screw Tripod top Altitude bubble Vernier arm Graduated arc Clamping nut Telescope Arm of vertical circle clamp Line of sight Axis of Plate bubble Lower Plate Tribrach Trivet Plumb bob

116 Lower plate/circle plate (8): an annular horizontal plate with the graduations provided all around, from 0 to 360, in a clockwise direction. The graduations are in degree divided in to 3 parts so that each division equals to 0 min. Horizontal angles are measured with this plate. The size of the theodolite is defined by the diameter of horizontal circle. Upper plate (7): Horizontal plate of smaller diameter provided with two verniers. on diametrically opposite parts of its circumference. These verniers are designated as A and B. They are used to read fractions of the horizontal circle plate graduations. The verniers are graduated in 0 min and each minute is divided in 3 to 5 parts making least count 0 or 0. Clamps and tangent screws (5, 9): There are two clamps and associated tangent screws with the plate. These screws facilitate the motion of the instruments in horizontal plane. Lower clamp screw locks or releases the lower plate. When this screw is unlocked both upper and lower plates move together. The associated lower tangent screw allows small motion of the plate in locked position. 6

117 The upper clamp screw locks or releases the upper vernier plate. When this clamp is released the lower plate does not move but the upper vernier plate moves with the instrument. This causes the change in the reading. The upper tangent screw allows the fine adjustment. Plate level (5): Spirit level with the bubble and graduation on glass cover. A single level or two levels fixed in perpendicular direction may be provided. The spirit level can be adjusted with the foot screw () of the levelling head (7). Telescope (0): The essential parts of the telescopes are eye-piece, diaphragm with cross hairs, object lens and arrangements to focus the telescope. Vertical circle (): circular plate supported on horizontal axis of the instrument between the A-frames. Vertical circle has graduation 0-90 in four quadrants. Vertical circle moves with the telescope when it is rotated in the vertical plane. Vertical circle clamp and tangent screw (): Clamping the vertical circle restrict the movement of telescope in vertical plane. Altitude level (): A highly sensitive bubble is used for levelling particularly when taking the vertical angle observations. 7