Setting out
Level-book calculation
- where
- height of instrument (m)
- reduced level (m)
- backsight (m)
- foresight (m)
Explain this equation
This reduces staff readings from an optical or laser level to a new reduced level using the height-of-instrument method. Use it to transfer a level from a benchmark to a new point.
RL_known is the reduced level of the benchmark or known point. BS, the backsight, is the staff reading taken on that known point. HI is the height of the instrument's line of sight. FS, the foresight, is the staff reading at the new point, and RL_new is its reduced level.
The staff measures down from the line of sight to the ground. So the line of sight sits BS above the known point, which gives HI, and the new point sits FS below the line of sight, which gives RL_new.
From a 100.000 m benchmark, a 1.200 backsight gives HI = 101.200. A 1.800 foresight puts the new point at 99.400, which is 600 mm lower. A bigger foresight always means a lower point.
Watch out: both readings must be from the same instrument set-up. Close back onto a known level to check your work.
Use a known reduced level and staff readings to find a new reduced level.
Enter your values to see the result.
Good to know
All readings must relate to the same instrument setup. Complete closing checks and record change points separately.
Sources
- Geometry / arithmetic
Estimates are not design approval or British Standards compliance checks. Structural arch, retaining-wall, wind, foundation, lintel and masonry-capacity design are not performed. Use the project engineer’s specification.