Levelling Applications: Art of measuring and calculating the height of a point relative to another point High accuracy is required. Measurement in a VP. Applications: We are used to measuring heights eg your height might be 1.83m or 1.55m The height of the seat you are sitting on might be 0.45m. Above the floor. On the other hand the height of the seat for you (4 ROWS BACK) would be 0.45m above your feet but 4*0.45m (1.80m) above my feet. So it’s important that we start thinking about height relative to some point. We could talk about each row of you being on a different level. We could use a spirit level and straight edge to find the different heights or levels. Ancient civilisations used Water levels. We could use the ceiling to measure up to and down from to get difference in height or level from the back row to the front row. That is OUR HORIZONTAL PLANE Show an OPTICAL LEVEL and TRIPOD Civil Engg Projects: Railways, Roadways, Bridges, Dams, Water Supply Schemes

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

Dumpy Level

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

Telescopic staff

CROSS HAIRS

Levelling up bubble centering Eliminations of parallax 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

SETTING UP

Levelling Up

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

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)

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.

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

Survey of India-MSL at Mumbai 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

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

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

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

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

BS Move instrument to new position RL A RL B RL C A B C S3 (CP) CP 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

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

Unknown RL = HPC - Fore Sight RL A RL B A B C RL C BS FS HPC = HPC 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

never move both Summary of Levelling Procedure When the level has been set up we always start with a BS to a point whose RL is known 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

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

Station B.S I.S F.S H.I R.L Remarks A 1.200   100.000 BM B 0.980 C 1.035 D 1.100 0.650 Change Point E 0.950 F 1.400 0.700 G 1.000 H 0.880 Sum

Check ∑BS-∑FS = Last RL – First RL

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.