Basic Surveying
Domain 4 tests Basic Surveying: chain and tape work, ranging, offsets, and levelling calculations. Numerical levelling questions are common and are lost through sign mistakes rather than lack of knowledge.
Chain and tape
- Ranging fixes intermediate points on a straight line with ranging rods; ranging rods are 2 m or 3 m long, painted in alternate bands.
- Offsets are perpendicular distances from the chain line to a detail; they are booked with the chainage.
- If a chain is longer than its nominal length, the measured distance is less than the true distance: true distance = measured × (actual length / nominal length).
Levelling
- A benchmark is a fixed point of known reduced level (RL); a temporary benchmark is set for the job.
- Height of instrument (HI) method: HI = RL of BM + backsight. RL of any point = HI − staff reading.
- Backsight is taken on a known point, foresight on the last point before shifting the instrument, intermediate sights on other points.
Worked example
BM RL = 100.000. BS = 1.500, so HI = 101.500. Intermediate sight 1.200 gives RL = 101.500 − 1.200 = 100.300. Foresight 0.900 gives RL = 101.500 − 0.900 = 100.600. Reasoning: all readings from one instrument station are subtracted from the same HI; a new HI is calculated only after the instrument moves.
Traps
- Adding the staff reading instead of subtracting it.
- Mixing up backsight and foresight, which reverses the height of instrument.
- Forgetting that a reading on an overhead point is inverted and taken as negative.