Work out survey levelling instantly with clear inputs, formula shown and shareable results.
The height of collimation method adds the backsight staff reading to the known benchmark level to get the instrument's line of sight, then subtracts each foresight to obtain the reduced level of that point. A larger foresight than backsight means the ground has fallen, because the staff had to be read further down.
Height of collimation
HI = benchmark RL + backsight reading
Reduced level
RL = HI - foresight reading
HI = benchmark RL + backsight reading. The height of collimation method adds the backsight staff reading to the known benchmark level to get the instrument's line of sight, then subtracts each foresight to obtain the reduced level of that point.
Rise and fall provides an arithmetic check on every intermediate sight and is preferred where accuracy matters. Height of collimation is faster when there are many intermediate points from one setup.
This calculator takes 3 inputs: Benchmark reduced level, Backsight reading on benchmark, Foresight reading at new point. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.