Surveying and Levelling
Scope of the domain
Surveying locates points on the ground; levelling determines their elevations. The exam tests survey principles, chain and compass work, bearings and level-book calculations.
Core rules and concepts
- Principle: work from whole to part; fix a new point by two independent measurements.
- Plane surveying treats the Earth as flat and is limited to small areas, about 250 km²; geodetic surveying accounts for curvature.
- Chain survey uses a chain or tape and offsets; ranging aligns intermediate points on a line. It suits small, fairly level areas.
- Bearings: whole-circle bearing (WCB) is measured clockwise from north from 0° to 360°; reduced bearing (RB) is measured from north or south toward east or west from 0° to 90°.
- Levelling: in the height of instrument (HI) method, HI = RL + BS; RL of the next point = HI − FS or IS. The rise-and-fall method compares consecutive staff readings.
Worked example
Benchmark RL = 100.000 m. Backsight on the benchmark = 1.250 m, so HI = 101.250 m. Foresight on a new point = 1.750 m, so RL = 101.250 − 1.750 = 99.500 m. Arithmetic check: ΣBS − ΣFS = 1.250 − 1.750 = −0.500 m; last RL − first RL = 99.500 − 100.000 = −0.500 m. This confirms the booking.
Traps
BS is taken on a point of known RL, not on the unknown point. FS and IS are subtracted from HI. If fore and back bearings of a line differ from 180°, local attraction or an observational error is present. Keep chainage and offset units consistent.