Surveying and Levelling
Surveying and Levelling tests field measurement of distances, directions, angles and elevations, and the reduction of field data into plans and levels. Expect chain and tape surveying, ranging, offsets, prismatic compass bearings, levelling, contouring and total-station basics.
Core procedures
- Chain surveying: suitable for small, open areas. Fix stations, range the line, measure with a chain or tape, and take offsets at right angles to the chain line.
- Compass surveying: bearings are measured from magnetic north. Whole-circle bearing is measured clockwise from 0° to 360°; reduce bearings to fore and back bearings. The difference between FB and BB should be 180° if local attraction is absent.
- Levelling: a level establishes a horizontal line of sight. Book readings as back sight, intermediate sight and fore sight. Reduce levels by the height-of-instrument method or rise-and-fall method.
- Total station: measures horizontal and vertical angles and slope distance electronically, then computes coordinates and reduced levels.
Worked example
A benchmark has RL 100.000 m. Back sight on the benchmark is 1.585 m, so height of instrument = 100.000 + 1.585 = 101.585 m. An intermediate sight of 1.150 m gives RL = 101.585 − 1.150 = 100.435 m. A fore sight of 1.045 m gives RL = 101.585 − 1.045 = 100.540 m.
Traps
Do not book IS and FS in the wrong column; keep the staff vertical and check parallax; do not ignore magnetic declination when converting magnetic bearings to true bearings; and always close a level run on a known benchmark to check misclosure.