Levelling

1. Levelling • Art of measuring and calculating the height of a point relative to another point • High accuracy is required. • Measurement in a VP. Applications: Civil Engg Projects: Railways, Roadways, Bridges, Dams, Water Supply Schemes

2. Levelling Instruments Level: A combination of telescope, level tube, levelling head and tripod Types: Dumpy Level Wye Level Reversible Level Tilting Level Automatic Level

5. Dumpy Level

6. 2. Levelling Staff • A graduated rod, wood or Al • Types: • Self Reading Type- Shorter distances • Target Type – Longer distances • Self Reading Type: • Smallest div. is 5mm • Each metre is 200div. • Black & White div. alternately • Numerals inverted, so that errect view through telescope • Types- a) Solid Staff • b) Telescopic Staff • c) Folding Staff

7. Telescopic staff

8. CROSS HAIRS

9. Adjustments of a level Temporary Permanent Temporary Setting up the level Levelling up bubble centering Eliminations of parallax 3.a) Focusing the eye piece 3.b) Focusing the objective

10. SETTING UP

12. Levelling Up

13. line of collimation diaphragm focusing screw eyepiece object lens object lens focusing lens focusing lens cross hairs Typical diaphragms - in different makes of instrument A surveying optical telescope Focusing 1. Rotate eyepiece to give a sharp, clear image of the cross hairs 2. Rotate focusing screw to give a sharp, clear image of the object being observed. The aim of focusing is to remove (eliminate) PARALLAX

14. Pond Bubble When pond 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) When the instrument is rotated the compensators ensure that a horizontal plane of collimation is swept out (hopefully)

15. Parallax When focussing any optical instrument it is vitally important that we eliminate Parallax. Move the eye up and down (or from left to right) over the eyepiece of the telescope. If the cross hairs move relative to the object being observed then Parallax exists and the focussing is not satisfactory.

16. Elimination of Parallax Focus the crosshairs (using the Eyepiece) Focus the object (using the Focussing screw)

17. Terminolgy • Mean Sea Level(MSL) • By making hourly observations of tides at any place over a period of 19 years. • Survey of India-MSL at Mumbai • RL or Elevation • The height or depth of a point above or below the assumed datum • Benchmark (BM): Any datum of known elevation

18. Level Field Book • For entering and reducing the readings(sights at various points). • Printed in a tabular form to facilitate booking of readings and reduction of levels. • Height of Instrument(H.I) or Height of Collimation (H.C) Method • Rise and Fall method

19. Levelling Staff S1 Reduced Level of B A B HPC = RL A + S1 DATUM Levelling Height of (HPC) the Plane of Collimation S2 RL A RL B RL B = HPC - S2

20. S1 RL C RL A RL B C A B Some Terminology BS Level staff on A Back Sight (BS) reading is first reading

21. S2 RL C RL A RL B C A B Level staff on A Back Sight (BS) reading is first reading Level staff on B FS Fore Sight (FS) reading is last reading Move instrument to new position

22. RL C RL A RL B C A B CP Move instrument to new position (CP) BS S3 Level staff stays on B The instrument has changed its position about point B Point B is known as a Change Point 2nd instrument position starts with BS to B

23. BS FS S3 S4 RL C RL A RL B C A B FS to C and finishes with

24. HPC HPC BS FS BS FS RL C RL A RL B C A B HPC = HPC = (CP) RL A is known RL A + BS RL B = HPC - FS Now the RL B is known So we can repeat the process RL B + BS RL C = HPC - FS Generally : HPC = Known RL + Back Sight Unknown RL = HPC - Fore Sight

25. When the level has been set up we always start with a BS to a point whose RL is known Summary of Levelling Procedure The last reading at any instrument position is always a FS Either the instrument moves or the staff moves - never move both We must always finish levelling at a point of known RL value always close your levelling

26. Height of Instrument Method • The following readings are taken on a level instrument with the station A as a bench mark with R.L 100.00m. The instruments is shifted after 4th and 7th reading. • Readings are given in a tabular form below. Find out the R.L

28. Check ∑BS-∑FS = Last RL – First RL

29. Problem • The following readings are taken on a level instrument with the station A as a bench mark with R.L 200.00m. • 2.190, 3.150, 1.060, 0.230, 3.430, 3.170, 3.420, 3.720, 2.390 • The instruments is shifted after the reading 3.430. Enter these readings in level book and calculate the R.L of all points.