Surveying and Site Setting Out
Surveying and Site Setting Out tests linear measurement, level transfer and setting a plan on the ground.
Chain survey and levelling
- Ranging: fixes intermediate points on a straight line; offsets are perpendicular distances from the chain line.
- Benchmark: a fixed point of known reduced level (RL). Height of instrument (HI) = RL of BM + backsight.
- RL of a point: HI − intermediate or foresight reading. A higher staff reading means a lower ground point.
- Setting out: use the 3-4-5 method for a right angle; check diagonals; mark corners with pegs and transfer levels with a dumpy level or boning rods.
Worked example
BM RL = 100.000 m. Backsight on the BM = 1.245 m. HI = 101.245 m. Foresight on a peg = 1.875 m. RL of peg = 101.245 − 1.875 = 99.370 m. If the peg must be at 99.500 m, it is 0.130 m too low; add 130 mm of filling or drive a new peg. Reason: the foresight is subtracted from HI.
Traps
Adding instead of subtracting the foresight; treating HI as an RL; ignoring inverted staff readings; not closing the level run; and setting out diagonals that do not match. Record backsight, intermediate and foresight in a level book.