Surveying, Setting Out and Levelling
This domain tests chain and compass basics, levelling bookkeeping with arithmetic checks, and setting out a building accurately.
Chain and compass work
Base line, ranging rods, tape and offsets taken at right angles with a cross staff or optical square. Whole-circle bearing (WCB) is measured clockwise from north; reduced bearing (RB) lies between 0° and 90° with a quadrant label; back bearing differs from fore bearing by 180°.
Levelling
Dumpy or auto level, tripod, levelling staff, benchmark. Two booking methods: height of instrument (HI) and rise and fall. Checks: ΣBS − ΣFS = last RL − first RL; and Σrise − Σfall equals the same value.
Setting out
Fix centre lines from the boundary or grid; set a right angle by the 3-4-5 method; mark pegs with nails; use batter boards for excavation; transfer levels with the level and staff; verify the rectangle by measuring both diagonals.
Worked example
BM RL 100.000, BS 1.500 gives HI 101.500. An intermediate sight of 2.000 gives RL 99.500. A foresight of 0.500 gives RL 101.000. Check: ΣBS − ΣFS = 1.500 − 0.500 = 1.000; last RL − first RL = 101.000 − 100.000 = 1.000. The booking is correct.
Traps
Adding instead of subtracting intermediate and fore sights; skipping the arithmetic check; reading the staff without centring the bubble; using a staff on soft ground without a base plate; setting out from one corner only without checking diagonals; long or very short sights that magnify error.