Setting out
Level-book calculation
- where
- height of instrument (m)
- reduced level (m)
- backsight (m)
- foresight (m)
Explain this equation
Reduces a pair of staff readings in a level book. From a known reduced level, a backsight and a foresight it gives the height of the instrument and the new reduced level.
RL is a reduced level, a height above an agreed datum, in metres. BS is the backsight: the staff reading on the known point. HI is the height of the instrument's line of sight. FS is the foresight: the staff reading on the new point.
The staff measures down from the line of sight to the ground. Adding the backsight to the known level gives the height of that line, HI. Subtracting the foresight from HI gives the level of the new point. This is the height-of-collimation method.
Known RL 100.000, BS 1.200 and FS 1.800 give HI 101.200 and a new RL of 99.400. A bigger foresight means the new point is lower.
Watch out: every reading must come from the same instrument set-up. Close the run back on a known level to check it.
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
This is a calculation aid, not a structural design or compliance certificate. Every default is an example, not a specification for your project.