Civil Engineering Department SURVEYING 1 CE 215 CHAPTER -3- LEVEL AND LEVELING 1 CHAPTER -3- LEVEL AND LEVELING 2 1
CONTENTS 1. Level instrument 2. Bubble 3. Tripod 4. Leveling staff 5. Definitions 6. Reading a staff 7. Finding the target 8. Basic rules for leveling 9. Field notes 10. Procedure of leveling 11. Leveling errors 12. Types of leveling 3 1. Level instrument 2
1. Base Plate 2. Horizontal Circle 3. Eyepiece 4. Circular Bubble 5. Sighting Pointer 6. Objective Lens 7. Focusing Knob 8. Fine Motion Drive 9. Footscrew 10. Bubble Mirror Level instrument 5 BASIC COMPONENTS OF LEVEL: 1. Telescope to provide a line of sight 2. Level Tube to make line of sight horizontal 3. Levelling head to bring the bubble of tube level at the centre of its run. 4. Tripod to support the above three parts of the level. 6 3
7 Leveling of the instrument is done to make the vertical axis of the instrument truly vertical. It is achieved by carrying out the following steps: Step 1: The level tube is brought parallel to any two of the foot screws, by rotating the upper part of the instrument. Step 2: The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward. (The bubble moves in the same direction as the left thumb.) Step 3: The level tube is then brought over the third foot screw again by rotating the upper part of the instrument. Step 4: The bubble is then again brought to the centre of the level tube by rotating the third foot screw either inward or outward. 4
Level Instrument 2. Bubble When bubble is centred the instrument s standing axis is approximately vertical. The compensators in the instrument take over and adjust the optical Line of Collimation so that it is horizontal (hopefully). 10 5
Keep the pole upright Any tilt will disturb your readings 11 3. Tripod From easy to sit to ops, this is high 12 6
Tripod stand :- is a portable three-legged frame, used as a platform for supporting the weight and maintaining the stability of some other object. TRIPOD STAND 13 4. Leveling staff Leveling Staff :-Is a wooden or metal ruler one side runway to meters and centimeters. And is a ruler of solid wood 2, 3, 4, 5 meters in length and usually 4 meters. 14 7
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19 5. Definitions 1. Datum line (M.S.L.) :-Is the level (line), which are attributed to it points levels on the surface of the Earth. Which is the average sea level. 2. Reduced level (R.L) :- Is the high point from datum line. Or height or depth of a point above or below the assumed datum. 3. Benchmark (B.M.) :-Are fixed points information site and attributed placed in different places until you start racing them when conducting settlement. Or is a fixed reference point of known elevation. It may be of the following types; 4. Level Surface :- A level surface is a surface, all points of which are normal to the pull of gravity. I.e. a line at right angles to the string line on a freely suspended plumb bob. The open sea provides a good example of a level surface. 10
21 a. GTS Bench mark (Geodetic Triangulation Survey): These Bench marks are established by national agency like Survey of India. They are established with highest precision. Their position and elevation above MSL is given in a special catalogue known as GTS Maps ( 100 km. interval). b. Permanent Bench Mark : They are fixed points of reference establish with reference to GTS Bench mark (10 km. interval). c. Arbitrary Bench mark : These are reference points whose elevations are arbitrarily assumed. In most of Engineering projects, the difference in elevation is more important than their reduced levels with reference to MSL as given in a special catalogue known as GTS Maps ( 100 Km. interval). 22 11
5. Back sight (B.S.) :- Is the first reading taken after placing the device in any position so that we see the greatest possible number of points required to find the elevation. 6. Change Point (CP) :- The point on which both the foresight and back sight are taken during the operation of levelling is called change point. 7. Fore sight (F.S) :- Is the last reading taken before the transfer device. 8. Intermediate sight (I.S.) :- Is reading taken between the back sight and fore sight reading. 9. Elevation of line of sight (H.I) :- Is the imaginary vertical level determined by the line of sight to the amount of increase or decrease for sea level (Height of Instrument). Measures height differences between points. Leveling rods Line of sight Back sight Fore sight 24 12
Benchmarks Read the [m], [dm] & [cm] Estimate the [mm] 6. Reading a Staff 1.422 Check yourself for frequent used numbers (2/3) or (7/8) 26 13
7. Finding the Target Most telescopes have a targeting system along the top of the barrel (similar to a rifle target finder). The telescope should be aimed at the staff using this targeting system prior to trying to find the staff through the eyepiece. Staff Target finder (Look at this and beyond towards staff) Eye-piece Staff Target finder (In line with staff) Eye-piece MOVE TELESCOPE UNTIL TRIANGLE APPEARS OVER STAFF Telescope Telescope 27 8. Basic Rules for Leveling Always start and finish a leveling run on a Benchmark (BM or TGBM) and close the loops. Keep fore sight and back sight distances as equal as possible. Keep lines of sight short (normally < 50m). Never read below 0.5m on a staff (refraction). Use stable, well defined change points. Beware of shadowing effects and crossing waters. 28 14
9. Field Notes S2 New Benchmark NB1 New Benchmark NB2 Back Inter Fore Point 1.327 2.365 2.347 3.753 3.982 0.986 3.724 1.101 TH NB1 NB2 NB1 TH Tidal Hut TH S1 29 10. Procedure of leveling 1. The instrument must be check before use! 2. The instrument and level must be stable settled-up 3. The bubble tube must be leveled before the reading Beware of sun exposure (will wander) Ensure the instruments pendulum is in-limit 4. The instrument must be set up in the middle between two staffs Prevents curvature effects If impossible, use the same distances, but opposite for the next readings 5. You must not use the parallax screw between the backsight and foresight readings 30 15
6. Readings must be taken 30-50 cm above the ground Surface refractions Beware also of temperature gradients (inside/outside buildings). 7. Staff should be set up vertically 8. A change plate should be used 9. Leveling must be done in two opposite directions but the same line (beware of gravity gradients) 10. Staff should be calibrated, especially if INVAR 11. Be careful when crossing rivers (large water surfaces) Use same-time (mutual) observations Repeat it during different times of the day 31 11. Leveling Errors There are a large number of potential sources of errors in leveling. Many of these are only significant for precise leveling over long distances. For the short segments of leveling that will occur in connecting a TGBM to nearby benchmarks there are only four worth mentioning: a. Collimation Error b. Error due to Earth Curvature c. Error due to Parallax Error d. Error due to Refraction 32 16
a. Collimation Error Line of Collimation Imaginary line that passes through leveling instrument at Cross Hairs 33 Collimation Error 34 17
b. Parallax Error When using an optical instrument both the image and cross hairs can be focused- if either is imprecisely focused, the cross hairs will appear to move with respect to the object focused, if one moves one's head horizontally in front of the eyepiece. 35 Parallax Error 36 18
c. Curvature of the Earth Due to the curvature of the Earth, the line of sight at the instrument will deviate from a horizontal line as one moves away from the level. 37 12. Types of Levelling 1) Simple levelling 2) Differential levelling 3) Fly levelling 4) Profile levelling 5) Cross sectional levelling 6) Reciprocal levelling 38 19
1. Simple levelling:- When the difference in the elevation of two nearby points is required then simple levelling is performed. 36 2. Differential levelling:- Performed when the final point is very far from the initial point. 37 20
3. Fly levelling Performed when the work site is very far away from the bench mark. The surveyor starts by taking BS at BM and proceed towards worksite till he finds a suitable place for temporary BM. All works are done with respect to temporary BM. At the end of the day the surveyor comes back to original BM. This is called fly levelling. 38 4. Profile levelling:- Profile levelling, which yields elevations at definite points along a reference line, provides the needed data for designing facilities such as highways, railroads, transmission lines. Reduced levels at various points at regular interval along the line is calculated. After getting the RL of various points the profile is drawn. Normally vertical scale is much larger than horizontal scale for the clear view of the profile. 39 21
5. Reciprocal levelling:- When levelling across river is required then this method is applied to get rid of various errors. 40 44 22