Leveling Example
Perform differential leveling: from a known benchmark elevation and a backsight reading, compute the height of instrument; then find the elevation of a turning point from a foresight reading.
Concept
In differential leveling, the instrument (level) is set up between a known point and the point whose elevation is desired. Backsight (BS) is the rod reading on the known point; foresight (FS) is the rod reading on the new point. Height of instrument (HI) = elevation of BM + BS. Then elevation of the new point = HI − FS.
Notation: = backsight, = foresight, = height of instrument, = elevation. Percent grade = (rise or drop / horizontal distance) × 100.
Problem
A level is set up and a backsight of 4.25 ft is taken on a benchmark with elevation 520.50 ft. The level is then aimed at a turning point and a foresight of 6.80 ft is recorded.
Find:
- Height of instrument (HI).
- Elevation of the turning point.
Given
- Benchmark elevation: 520.50 ft
- Backsight (BS) on BM: 4.25 ft
- Foresight (FS) on turning point: 6.80 ft
Height of instrument
Elevation of turning point
Final Answer
(1) Height of instrument: HI = 524.75 ft.
(2) Elevation of turning point: 517.95 ft (below the BM because FS > BS).