Surveying and Site Setting Out
This domain tests field procedures: ranging a line, taking offsets, reducing levels from a benchmark, and transferring a right angle or a rectangular grid from drawing to ground.
Core procedures
- Ranging is direct when both ends are visible and indirect when an intermediate point is required; offsets are perpendicular or oblique, and cross-staff or prism square sets the perpendicular.
- Levelling uses a benchmark of known reduced level. Height of instrument HI = RL of backsight point + backsight reading. RL of any point = HI − staff reading on that point.
- The HI stays unchanged until the instrument is moved; a new backsight is then taken.
- Arithmetic check: sum of backsights − sum of foresights = last RL − first RL.
- Setting out a right angle uses the 3-4-5 method or the diagonal check; a rectangle is proved by equal diagonals, measured with a steel tape and corrected for slope and temperature.
Worked example
Benchmark RL 100.000 m, backsight 1.450 m, so HI = 101.450 m. A foresight of 1.875 m on the first point gives RL = 101.450 − 1.875 = 99.575 m. An intermediate sight of 2.100 m on a second point from the same instrument position gives RL = 101.450 − 2.100 = 99.350 m. If a peg at 3 m and a peg at 4 m on the two arms are exactly 5 m apart on the diagonal, the corner is square.
Traps that cost marks
Adding the foresight instead of subtracting it; changing the HI at every reading without moving the instrument; applying the 3-4-5 check along a sloping ground without correcting for slope; ignoring tape sag and temperature on long baselines; recording readings in the field book without the backsight, intermediate and foresight columns labelled.