Levelling and Contouring
Levelling and Contouring
This domain tests height control: benchmark to reduced level, backsight and foresight, rise-and-fall versus height-of-instrument (HI) booking, and how contours represent terrain. Numerical questions are common.
Core rules
- Benchmark (BM) has a known reduced level (RL). Backsight (BS) is the first staff reading after setting up; foresight (FS) is the last reading before moving; intermediate sights (IS) are other readings.
- HI method: HI = RL + BS. RL of a point = HI - staff reading. The instrument height stays the same until the instrument moves.
- Rise-and-fall method: rise = previous staff reading - next staff reading; fall = next reading - previous reading.
- Arithmetic check: sum of BS - sum of FS = last RL - first RL for a level run.
- Combined curvature and refraction correction is about 0.0673 D² metres, with D in km; it is subtractive from the staff reading. Curvature alone is 0.0785 D² m.
- Contour interval is the vertical distance between successive contours; it is constant on one map. Closely spaced contours mean steep ground; widely spaced mean gentle ground.
Worked example
BM RL = 100.000 m. BS = 1.500 m, so HI = 101.500 m. FS on point P = 2.200 m, so RL(P) = 101.500 - 2.200 = 99.300 m. Arithmetic check: BS - FS = 1.500 - 2.200 = -0.700 m, and last RL - first RL = 99.300 - 100.000 = -0.700 m. The booking balances.
Traps
Do not add a foresight to HI; always subtract. Do not apply curvature correction with the wrong sign or with D in metres instead of kilometres. A contour interval is not a horizontal distance. Interpolation between contours assumes a uniform slope, which is an approximation.