UNIT-2 COMPASS SYRVEYING AND PLANE TABLE SURVEYING THE PRISMATIC COMPASS Prismatic compass is the most convenient and portable of magnetic compass which can either be used as a hand instrument or can be fitted on a tripod. The magnetic needle is attached to the circular ring or compass card made up of aluminium, a non- magnetic substance. When the needle is on the pivot it will orient itself in the magnetic meridian and, therefore, the N and S ends of thc ring will be in this direction. The line of sight is defined by the objective vane and the eye slit, both attached to the compass box. The object vane consists of a vertical hair attached to a suitable frame while the eye slit consists of a vertical slit cut into the upper assembly of the prism unit, both being hinged to the box. When an object is sighted, the sight vanes wilt rotate with rcspectto the NS end of ring through an angle which the line makes with the magnetic meridian. A triangular prism is fitted below the eye slit having suitable arrangement for focusing to suit different eye sights.the prism has both horizontal and vertical faces convex, so that a magnified image of the ring graduation is formed. When the line of sight is also in the magnetic meridian, the South end ring comes vertically below the horizontal face of the prism. The 0 or 360 reading is, therefore, engraved on the South end of the ring, so that bearing of the magnetic meridian is read as 0. The object vane presses against a bent lever which lifts the needle off the pivot and holds it against the glass lid. By pressing knob or break pin placed at the base of the object vane, a light spring fitted inside the box can be brought into the contact with the edge of the graduated ring to damp the oscillations of the needle when about to take the reading. The greatest advantage of prismatic compass is that both sighting he object as well as reading circle can be done simultaneously without hanging the position of the eye. The circle is read at the reading at which the hair line appears to cut the graduated ring. Adjustment of Prismatic compass Station or Temporary Adjustments Centring : Centring is the process of keeping the instrument exactly over the station. Levelling: If the instrument is a hand instrument, it must in hand in such a way that graduated disc is swinging freely appears to be level as judged from the top edge of the ease. Focusing the Prism: The prism attachment is slided up or down for focusing till the readings are seen to be sharp and clear.
Permanent Adjustments The permanent adjustments of prismatic compass are almost the same as that of the surveyor s except that there are no bubble tubes to be adjusted and the needle, cannot be straightened. The sight vanes are generally not adjustable THE SURVEYOR S COMPASS Fig. shows the essential parts of a surveyor s compass. The graduated ring is directly attached to the box and not with needle. The edge bar needle freely floats over the pivot. Thus, the graduated card or ring is not oriented in the magnetic meridian, as was the case in the prismatic compass. The object vane is similar to that of prismatic compass. The eye vane consists of a simple metal vane with a fine slit Since no prism is provided, the object is to be sighted first with the object and eye vanes and the reading is then taken against the North end of the needle, by looking vertically through the top glass.
When line of sight is in magnetic meridian, the North and south ends of the needle will be over the 0 N and 0 S graduations The card is graduated in quadrantal system having 0 at N and S ends and 90 East and West ends. Let us take the case of a line AB which is in North-East quadrant. In order to sight the point B, the box will have to be rotated about the vertical axis In doing so, the pointer of the needle remains fixed in position while 0 N graduation of the card moves in a clockwise direction. Taking when the line has a bearing of 90 in East direction, the ponter appears to move by 90 from the 0 N graduation in anti-clockwise direction
Difference Between Primatic Compass & Surveyor s Compass BEARING. Bearing of a line is its direction relative to a given meridian. A meridian is any direction such as : True meridian Magnetic Meridian Arbitrary Meridian.
(1) True Meridian. The meridian through a point is the line in which.a plane, passing that point and the north and south pd intersects with surface of the earth. It, thus, passes through the north and south. True Bearing. True bearing of a line is the horizontal angle which it makes with the true meridian through one of the extremities of the line. Since the direction of true meridian through a point remains fixed, the true bearing of a line is a constant quantity. (2) Magnetic Meridian Magnetic meridian through a is the direction shown by a freely floating and balanced magnetic needle free from all other attractive forces. Magnetic Bearing The magnetic bearing of a line is the horizontal angle which it makes with the magnetic meridian passing through one of the extremities of the line. (3) Arbitrary Meridian.Arbitrary meridian is any convenient direction towards a permanent and prominent mark or signal, such as a church spire or top of a chimney. Arbitrary bearing Arbitrary bearing of a line is the horizontal angle which it makes with any arbitrary meridian passing through one of the extremities. Conversion of W.C.B. into R.B. LINE W.C.B Rule for R.B. Quadrant AB 0 & 90 R.B.=W.C.B NE AC 90 & 180 R.B.= 180 -W.C.B SE AD 180 & 270 R.B.=W.C.B.- 180 SW AF 270 & 360 R.B.= 360 -W.C.B. NW Conversion of R.B. into W.C.B. LINE R.B. Rule for W.C.B. W.C.B between AB N E W.C.B= R.B 0 & 90 AC S E W.C.B.= 180 - R.B 90 & 180 AD S W W.C.B.=180 +R.B. 180 & 270 AF N ø W W.C.B.= 360 - R.B 270 & 360 CALCULATION OF ANGLES FROM BEARINGS v Knowing the bearing of two lines, the angle between the two can be easily calculated with the help of a diagrams v The included angle between the lines AC and AB = 1 & 2 = F.B. of one line F.B. of the other line, both bearings being measured from a common point A. Refer Fig. the angle = (180 + 1 )- 2 = B. B. of previous line. F.B. of next line.
- Let us consider the quadrantal bearing. If the bearings have been measured to the same side of common meridian, the include angle = 2-1 In fig (b) both the bearings have been measured to the opposite sides of the common meridian, and included angle = 1 + 2 In fig both the bearings have been measured to the same side of different and the included angle meridian, and included angle = 180-2 + 1 ) In fig (d) both the bearings have been measured to the opposite sides of different meridians, and angles = 180-1 - 2 ) CALCULATION OF BEARINGS FROM ANGLES
Let,,, be the included angles measured clockwise from back stations & 1 be the measured bearing of the line AB. The bearing of the next line BC = 2 = 1-180 The bearing of the next line CD = 3 = 2 + - 180 The bearing of the next line DE= 4 = 3 + - 180 The bearing of the next line EF= 5 = 4 + +- 180 1 ), ( 2 + ), ( 3 + ) are more than 180 while ( 4 + ) is less than 180. Therefore in order to calculate yhe bearing the following statement can be made: Add the measured clockwise angles to the bearing of the previous line. If the sum is more than 180, deduct 180. If the sum is less than 180, add 180 EXAMPLES ON ANGLES AND BEARINGS Example : Convert the following whole circle to quadrantal bearings (i) 22 30 (ii) 170 º 12 (iii) 211 54 (iv) 327 24. (b) Convert the following quadrantal bearing to whole circle bearings (i) N12 24 E (ii) S31 36 E (iii) S 68 6 W (iv) N5 42 W Referring to fig above and tables given: (i) R.B.= W.CB =22 30 =N22 30 E (ii) R.B.= 180 W. C. B. =180-170 12 =S 9 48 E (iii) R.B.= W. C. B. 180 =211 54 180 =S 31 54 W (iv) R.B.= 360 W.C.B.=360 327 24 =N 32 36 W (i)wcb= RB=12 24 (ii) WCB = 180 RB = 180 31 36 =148 º 24 (iii)w.c.b.= 180 + R.B.= 180 + 68 6 = 248 º 6 (iv)w.c.b.= 360 R.B. = 360 5 42 = 354 18
EARTH S MAGNETIC FIELD AND DIP v The horizontal projections of the lines of force define the magnetic meridian. The angle which these lines of force make with the surface of the earth is called the angle of dip or simply the dip of the needle MAGNETIC DECLINATION v Magnetic declination at a place is the horizontal angle bet the true meridian and the magnetic meridian shown by the ne at the time of observation. v If the magnetic meridian is to the right side (or eastern side) of the true meridian, declination is said to be eastern or positive, if it to be the left side (or western side), the declination is said to be western or negative.
Plane Table Surveying Definition Plane tabling is a graphical method of survey in which the field observations and plotting proceed simultaneously. v It is means of making a manuscript map in the field while the ground can be seen by the topographer and without intermediate steps of recording and transcribing field notes. v It can be used to tie topography by existing control and to carry its own control systems by triangulation or traverse and by lines of levels. Instruments used The following instruments are used in plane table survey 1. The plane table with levelling head having arrangements for (a) levelling, (b) rotation about vertical axis, and (c) clamping in any required position. 2. Alidade for sighting 3. Plumbing fork and plumb bob. 4. Spirit level. 5. Compass. 6. Drawing paper with a rainproof cover. 1. The Plane Table Three distinct types of tables having devices for levelling the plane table and controlling its Orientation are in common use : Traverse Table The traverse table consists of a small drawing board mounted on a light tripod in such a way that the board can be rotated about the vertical axis and can be clamped in any position. The table is levelled by adjusting tripod legs, usually by eye-estimation. Johnson Table This consists of a drawing board usually 45x60cm or 60x75 cm. The head consists of a ball-and-socket joint and a vertical spindle with two thumb screws on the underside. The ball-and-socket joint is operated by the upper thumb screw. When the upper screw is free, the table may be tilted about the ball-and socket for levelling. The clamp is then tightened to fix the board in a horizontal position. When the lower screw is loosened, the table may be rotated about the vertical axis and can thus be oriented. The Coast Survey Table The table is superior to the above two types and is generally used for work of high precision. The levelling of the table is done very accurately with the help of the three foot screws. The table can be turned about the vertical axis and can be fixed in any direction very accurately with the help of a clamp and tangent screw.
2.Alidade A plane table alidade is a straight edge with some form of sighting device. Two types are used : (i) Plain alidade (ii) Telescopic alidade. Plain Alidade. v It is used for ordinary work. v It generally consist of a metal or wooden rule with two vanes at the ends. v The two vanes or sight are hinged to fold down on the rule when the alidade is not in use. v One of the vanes is provided with a narrow slit while the other is open and carries a hair or thin wire. Both the slits thus provide a definite line of sight which can be made to pass through the object to be sighted. v The alidade can be rotated about the point representing the instrument station on the sheet so that the line of sigh passes through the object to be sighted. v A line is then drawn against the working edge (known as the fiducial edge) of the alidade. v It is essential to have the vanes perpendicular be the surface of the sheet. v The alidade is not very much suitable on hilly area since the inclination of the line of sight is limited. v A string joining the tops of the two vanes is sometimes provided to use it when sights of considerable inclination have to be taken. Telescopic Alidade. v The telescopic alidade is used when it is required to take in lined sights. v Also the accuracy and range of sights are increased by its use. v It essentially consists of a small telescope with a level tube and graduated arc mounted on horizontal axis. v The horizontal axis rests on a A-frame fitted with vernier fixed in position in the same manner as that in a transit. v All the parts are finally supported on a heavy rule, one side of which is used as the working edge along which line may be drawn. The inclination of the line of sight can be read on the vertical circle. v The horizontal distance between the instrument and the point sighted can be computed by taking stadia readings on the staff kept at the point. v The elevation of the point can also be computed by using usual tacheometric relations. v Sometimes, to facilitate calculation work, a Beaman stadia are may be provided as an extra. v Thus, the observer can very quickly and easily obtain the true horizontal distance from the plane table to a levelling staff placed at the point and the difference in elevation between them. v The same geometric principle apply to the alidade as to the transit, but the adjustments are somewhat modified in accordance with the lower degree of accuracy required.
3. Plumbing Fork v The plumbing fork is used in large scale work, is meant for centring the table over the point or station occupied by the plane table when the plotted position of that point is already known on the sheet. v In the beginning of the work it is meant for transferring the ground point on to the sheet so that the plotted point and the ground station are in the same vertical line. v The fork consists of a hair pin-shaped light metal frame having arms of equal length, in which a plumb-bob is suspended from the end of the lower-arm. v The fitting can be placed with the upper arm lying on the top of the table and the lower arm below it. v The table being centred when the plumb-bob hangs freely over the ground mark and the pointed end of the upper arm coincides with the equivalent point on the plan. 4. Spirit Level v A small spirit level may be used for ascertaining if the table is properly level. v The level may be either of the tubular variety or of the circular type, essentially with a flat base so that it can be laid on the table and is truly level when the bubble is central. v The table is levelled by placing the level on the board in two positions at right angles and getting the bubble central in both positions. 5. Compass v The compass is used for orienting the plane table to magnetic north.
v The compass used with a plane table is a trough compass v In which the longer sides of the trough are parallel and flat so that either side can be used as a ruler or laid down to coincide with a straight line drawn on the paper. 6. Drawing Paper v The drawing paper used for plane tabling must be of superior quality so that it may have minimum effect of changes in the humidity of the atmosphere. v The changes in the humidity of the atmosphere produce expansion and contraction in different directions and thus alter the scale and distort the map. v To overcome this difficulty, sometimes two sheets are mounted with their grains at right angles and with a sheet of muslin between them. v Single sheet must be seasoned previous of the use by exposing it alternatively to a damp and a dry atmosphere. v For work of high precision, fibre glass sheets or paper backed with sheet aluminium are often used. WORKING OPERATIONS Three operations are needed (a) Fixing : Fixing the table to the tripod. (b)setting : (i)levelling the table (ii)centring (iii)orientation. ( c) sighting the points Levelling v For small-scale work, levelling is done by estimation. v For work of accuracy, an ordinary spirit level may be used. v The table is levelled by placing the level on the board in two positions at right angles and getting the bubble central in both directions. v For more precise work, a Johnson Table or Coast Survey Table may be used. Centring v The table should be so placed over the station on the ground that the point plotted on the sheet corresponding to the station occupied should be exactly over the station on the ground. v The operation is known as centring the plane table. Orientation v Orientation is the process of putting the plane-table into some fixed direction so that line representing a certain direction on the plane is parallel to that direction on the ground. v If orientation is not done, the table will not be parallel to itself at different positions resulting in an overall distortion of the map. v The processes of centring and orientation are dependent on each other. v For orientation, the table will have to be rotated about its vertical axis, thus disturbing the centring. v If precise work requires that the plotted point should be exactly over the ground point, repeated orientation and shifting of the whole table are necessary. There are two main methods of orienting the plane table (i) Orientation by means of trough compass.
(ii) Orientation by means of back sighting (i) Orientation by trough compass The plane table can be oriented by compass under the following conditions (a) When speed is more important that accuracy. (b) When there is no second point available for orientation. (c) When the traverse is so long that accumulated errors in carrying the azimuth forward might be greater than orientation by compass. (d) For approximate orientation prior to final adjustment (e) In certain resection problems. (ii) Orientation by back sighting Orientation can be done precisely by sighting the points already plotted on the sheet. Two cases may arise (a) When it is possible to set the plane table on the point already plotted on the sheet by way of observation from previous station. (b) When it is not possible to set the plane table on the point. METHODS OF PLANE TABLING Methods of plane tabling can be divided into four distinct 1. Radiation. 2. Intersection. 3. Traversing. 4. Resection. The first two methods are generally employed for locating the details while the other two methods are used for locating the plane table stations. table stations. RADIATION In this method, a ray is drawn from the instrument station towards the point, the distance is measured between the instrument station and that point, and the point is located by plotting to some scale the distance so measured. Evidently, the method is more suitable when the distances are small and one single instrument can control the points to be detailed. The method has a wider scope if the distances are obtained tacheometrically with the help of telescopic alidade.
The following steps are necessary to an instrument station to locate the points from an instrument station: 1. Set the table at T, level it and transfer the point on to the sheet by means of plumbing fork, thus getting point t representing T. Clamp the table. 2. Keep the alidade touching t and sight to A. Draw the ray along the fiducial edge of the alidade. Similarly, sight different points B, C, D, E etc., and draw the corresponding rays. 3. Measure TA, TB, TC, TD, TE etc., in the field and plot their distances to some scale along the corresponding rays, thus getting a, b, c, d, e etc. Join these if needed. INTERSECTION (GRAPHIC TRIANGULATION) Intersection is resorted to when the distance between the point and the instrument station is either too large or cannot be measure accurately due to some field conditions. The location of an object is determined by sighting at the object from two plane table stations and drawing the rays. The intersection of these rays will give the position of the object. It is therefore very essential to have at least two instrument stations to locate any point. The distance between the two instrument stations is measured and plotted on the sheet to some scale. The line joining the two instrument stations is known as the base line. No linear measurement other, than that of the base line is made. The point of intersectior of the two rays forms the vertex of a triangle having the two rays as two sides and the base line as the third line of the triangle. Due to this reason, intersection is also sometimes known as graphic triangulation.
Procedure The following is the procedure to locate the points by the method of intersection: (1) Set the table at A, level it and transfer the point A on to the sheet by way of plumbing fork. Clamp the table. (2) With the help of the trough compass, mark the north direction on the sheet. (3) Pivoting the alidade about a, sight it to B. Measure AB and plot it along the ray to get b. The base line ab is thus drawn. (4) Pivoting the alidade about a, sight the details C, D, E etc, and draw corresponding rays. (5) Shift the table at B and set it there. Orient the table roughly by compass and finally by back sighting A. (6) Pivoting the alidade about b, sight the details C, D, E etc. and draw the corresponding rays along the edge of the alidade to intersect with the previously drawn rays in c, d, e etc. The positions of the points are thus mapped by way of intersection. TRAVERSING v Plane table traverse involves the same principles as a transit traverse. v The only difference is that in the case of radiation the observations are taken to those points which are to be detailed or mapped while in the case of traversing the observations are made to those points which will sub sequently be used as instrument stations. Procedure. (1) Set the table at A. Use plumbing fork for transferring A on to the sheet. Draw the direction of magnetic meridian with the help of trough compass. (2) With the alidade pivoted about a, sight it to B and draw the ray. Measure AB and scale off ab to some scale. Similarly draw a ray towards E, measure AE and plot e.
(3) Shift the table to B and set it. Orient the table accurately back sighting A. Clamp the table. (4) Pivoting the alidade about b, sight to C. Measure BC and plot it on the drawn ray to the same scale. Similarly, the table can be set at other stations and the traverse is completed. (5) It is to be noted here that the orientation is to be done by back sighting (6) If there are n stations in a closed traverse, the table will have to be set on at least (n 1) stations to know the error of closure though the traverse may be closed even by setting it on (n 2) stations. RESECTION Resection is the process of determining the plotted position of the station occupied by the plane table, by means of sights taken towards known points, locations of which have been plotted. The following are the four methods of orientation: (i) Resection after orientation by compass. (ii) Resection after orientation by back sighting. (iii) Resection after orientation by three-point problem. (iv) Resection after orientation by two-point problem. Resection after orientation by compass v The method is utilized only for small-scale or rough mapping for which the relatively large errors due to orienting with the compass needle would not impair the usefulness of the map.
(1) Let C be the instrument station to be located on the plan. Let A and B be two visible stations which have been plotted on the sheet as a and b. Set the table at C and orient it with compass. Clamp the table. (2) Pivoting the alidade about a, draw a resector (ray) towards A; similarly, sight B from b and draw a resector. The intersection of the two resectors will give c, the required point. of the two resectors will give c, the required point. Resection after orientation by backsighting If the table can be oriented by backsighting along a previously plotted backsight line, the station can be located by the intersection of the backsight line and the resector drawn through another known point.
(1) Let C be the station to be located on the plan and A and B be two visible points which have been plotted on the sheet as a and b. Set the table at A and orient it by backsighting B along ab. (2) Pivoting the alidade at a, sight C and draw a ray. Estimate roughly the position of C on this ray as c1 (3) Shift the table to C and centre it approximately with respect to c. Keep the alidade on the line c1 a and orient the table by back-sight to A. Clamp the table which has been oriented. (4) Pivoting the alidade about b, sight B and draw the resector bb to intersect the ray C1a in c. Thus, c is the location of the instrument station. Resection by Three-point Problem and Two-point Problem v Of the two methods described above, the first method is rarely used as the errors due to local attraction etc., are inevitable. v In the second method, it is necessary to set the table on one of the known points and draw the ray towards the station to be located. In the more usual case in which no such ray has been drawn, the data must consist of either (a) Three visible points and their plotted positions (The three- point problem). (b) Two visible points and their plotted positions (The two point problem). THE THREE-POINT PROBLEM Statement: Location of the position, on the plan, of the station occupied by the plane table by means of observations to three well-defined points whose positions have been previously plotted on the plan In other words, it is required to orient the table at the station with respect to three visible points already located on the plan. Let P be the instrument station and A, B, C be the points which are located as a, b, c respectively on the plan. The table is said to be correctly oriented at P when the three resectors through a, b and c meet at a point and not in a triangle. The intersection of the three resectors in a point gives the location of the instrument station. Thus, in three-point problem, orientation and resection are accomplished in the same operation.
The following are some of the important methods available for the solution of the problem (a) Mechanical Method (Tracing Paper Method) (b) Graphical Method (c) Lehmann s Method (Trial and Error Method) 1. MECHANICAL METHOD (TRACING PAPER METHOD) The method involves the use of a tracing paper and is, there- fore, also known as tracing paper method.
Procedure Let A, B, C be the known points and a, b, c be their plotted positions. L P be the position of the instrument station to be located on the map. (1) Set the table on P. Orient the table approximately with eye so that ab is parallel to AB. (2) Fix a tracing paper on the sheet and mark on it p as the approximate location of P with the help of plumbing fork. (3) Pivoting the alidade at p, sight A, B, C in turn and draw the corresponding lines p a, p b and p c on the tracing paper. These lines will not pass through a, b, and c as the orientation is approximate. (4) Loose the tracing paper and rotate it on the drawing paper in such a way that the lines p a, p b and p c pass through a, b and c respectively. Transfer p on to the sheet and represent it as p. Remove the tracing paper and join pa, pb and pc. (5) Keep the alidade on pa. The line of sight will not pass through A as the orientation has not yet been corrected. To correct the orientation, loose the clamp and rotate the plane table so that the line of sight pass through A. Clamp the table. The table is thus oriented. (6) To test the orientation, keep the alidade along pb. If the orientation is correct, the line of sight will pass through B. Similarly, the line of sight will pass through C when the alidade is kept on pc. 2. GRAPHICAL METHODS There are several graphical methods available, but the method given by Bessel is more suitable and is described first. Bessel s Graphical Solution (1) After having set the table at station P, keep the alidade on ba and rotate the table so that A is bisected. Clamp the table. (2) Pivoting the alidade about b, sight to C and draw the ray xy along the edge of the alidade fig (a) (3) Keep the alidade along ab and rotate the table till B is bisected. Clamp the table. (4) Pivoting the alidade about a, sight to C. Draw the ray along the edge of the alidade to intersect the ray xy in c fig. (b) Join cc (5) Keep the alidade along C C and rotate the table till C is bisected. Clamp the table. The table is correctly oriented fig (c). (6) Pivoting the alidade about b, sight to B. Draw the ray to intersect cc in p. Similarly, if alidade is pivoted about a and A is sighted, the ray will pass through p if the work is accurate.
The points a, b, c and p form a quadrilateral and all the four points lie along the circumference of a circle. Hence, this method is known as Bessel s Method of Inscribed Quadrilateral. LEHMANN S METHOD Procedure: (1) Set the table at P and orient the table approximately so that ab is parallel to AB. Clamp the table. (2) Keep the alidade pivoted about a and sight A. Draw the ray. Similarly, draw rays from b and c towards B and C respectively. If the orientation is correct, the three rays will meet at one point. If not, they will meet in three points forming one small triangle of error. (3) The triangle of error so formed will give the idea for the further orientation. The orientation will be correct only when the triangle of error is reduced to one point. To do this, choose the point p as shown. The approximate choice of the position may be done with the help of Lehmann s Rules described later. (4) Keep the alidade along p a and rotate the table to sight A. Clamp the table. This will give next approximate orientation (but more accurate than the previous one). (5) Keep the alidade at b to sight B and draw the ray. Similarly, keep the alidade at c and sight C. Draw the ray. These rays will again meet in one triangle, the size of which will be smaller than the previous triangle of error, if p has been chosen judiciously keeping in the view the Lehmann s Rules.
(6) Thus, by successive trial and error, the triangle of error can be reduced to a point. The final and correct position of the table will be such that the rays Aa, Bb and Cc meet in one single point, giving the point p. The whole problem, thus, involves a fair knowledge of Lehmann s Rules for the approximate fixation of p so that the triangle of error may be reduced to a minimum. The lines joining A, B, C (or a, b, c) form a triangle known as the Great Triangle. Similarly, the circle passing through A, B, C or (a, b, c) is known as the Great Circle. TWO-POINT PROBLEM Statement: Location of the position on the plan, of the station occupied by the plane table by means of observation to two well defined points whose position have been previously plotted on the plan. Let us take two points A and B, the plotted positions of which are known. Let C be the point to be plotted. The whole problem is to orient the table at C. Procedure (1) Choose an auxiliary point D near C, to assist the orientation at C. Set the table at D in such a way that ab is approximately parallel to AR (either by compass or by eye judgment). Clamp the table. (2) Keep the alidade at a and sight A. Draw the resector. Similarly, draw a resector from b and B to intersect the previous one in c The position of d is thus got, the degree of accuracy of which depends upon the approximation that has been made in keeping at parallel to AR. Transfer the point d to the ground and drive a peg.
(3)Keep the alidade at d and sight C. Draw the ray. Mark a point c on the ray by estimation to represent the distance DC. (4) Shift the table to C, orient it (tentatively) by taking backsight to D and centre it with reference to c The orientation is, thus, the same as it was at D. (5)Keep the alidade pivoted at a and sight it to A. Draw the ray to intersect with the previously drawn ray from D in c. Thus, c is the point representing the station C, with reference to the approximate orientation made at D. (6)Pivoting the alidade about c, sight B. Draw the ray to intersect with the ray drawn from D to B in b. Thus b is the approximate representation of B with respect to the orientation made at D. (7)The angle between ab and ab is the error in orientation and must be corrected for. In order that ab and ab may coincide (or may become parallel) keep a pole P in line wih ab and at a great distance. Keeping the alidade along ab, rotate the table till P is bisected. Clamp the table. The table is thus correctly oriented. (8) After having oriented the table as above, draw a resector from a to A and another from b to B, the intersection of which will give the position C occupied by the table. ERRORS IN PLANE TABLING The degree of precision to be attained in plane tabling depends upon the character of the survey, the quality of the instrument, the system adopted and upon the degree to which accuracy is deliberately sacrificed for speed. The various sources of errors may be classified as 1. Instrumental Errors : Errors due to bad quality of the in strument. This includes all errors described for theodolite, if telescopic alidade is used. 2. Errors of plotting. 3. Error due to manipulation and sighting. These include (a) Non-horizontality of board. (b) Defective sighting. (c) Defective orientation. (d) Movement of board between sights. (e) Defective or inaccurate centring. (a) Non-horizontality of board The effect of non-horizontality of board is more severe when the difference in elevation between the points sighted is more.
(b) Defective sighting The accuracy of plane table mapping depends largely upon the precision with which points are sighted. The plain alidade with open sight is much inferior to the telescopic alidade in the definition of the line of sight. (c) Defective orientation Orientation done with compass is unreliable, as there is every possibility of local attraction. Erroneous orientation contribute to wards distortion of the survey. This orientation should be checked at as many stations as possible by sighting distant prominent objects already plotted. (d) Movement of board between sights Due to carelessness of the observer, the table may be disturbed between any two sights resulting in the disturbance of orientation. To reduce the possibility of such movement, the clamp should be firmly applied. It is always advisable to check the orientation at the end of the observation from a station. (e) Inaccurate centring It is very essential to have a proper conception of the extent of error introduced by inaccurate centring, as it avoids unnecessary waste of time in setting up the table by repeated trials.