Lecture 6 GEODESY Levelling Levelling v Art

Lecture 6 GEODESY Levelling

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

Levelling Instruments Level: A combination of telescope, level tube, levelling head and tripod Types: a) b) c) d) e) Dumpy Level Wye Level Reversible Level Tilting Level Automatic Level

Dumpy Level

2. Levelling Staff v. A graduated rod, wood or Al v. Types: • Self Reading Type- Shorter distances • Target Type – Longer distances Self Reading Type: • Smallest div. is 5 mm • Each metre is 200 div. • 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

Adjustments of a level a) Temporary b) Permanent Temporary 1. Setting up the level 2. Levelling up bubble centering 3. Eliminations of parallax 3. a) Focusing the eye piece 3. b) Focusing the objective

SETTING UP

Levelling Up

A surveying optical telescope diaphragm line of focusing screw collimation eyepiece object lens focusing lens cross hairs Typical diaphragms - in different makes of instrument

A surveying optical telescope diaphragm line of focusing screw collimation eyepiece focusing lens object lens 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

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

Levelling Staff Levelling Height of the Plane of Collimation (HPC) S 1 S 2 Reduced Level of B RL A A B HPC = RL B = RL A + S 1 HPC DATUM - S 2 RL B

Some Terminology BS S 1 RL A A Level staff on A RL C RL B C B Back Sight (BS) reading is first reading

FS S 2 RL A A RL C RL B C Level staff on A B Back Sight (BS) reading is first reading Level staff on B Fore Sight (FS) reading is last reading Move instrument to new position

BS S 3 RL A A RL C RL B CP Move instrument to new position Level staff stays on B The instrument has changed its position about point B Point B is known as a Change Point (CP) 2 nd instrument position starts with BS to B

BS S 3 RL A A FS S 4 RL C RL B C B and finishes with FS to C

HPC BS RL A A BS FS FS RL C RL B B (CP) C

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