RL Difference Calculator
Derives reduced levels from backsight and foresight readings using the height-of-collimation method.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the known RL and the backsight and foresight staff readings.
- Press Calculate.
- Read the new RL and the height difference.
How it works
New RL = known RL + backsight − foresight; the height of collimation = known RL + backsight. This is the rise-and-fall basis of differential levelling.
Worked example
RL 100.000, BS 1.250, FS 0.880. New RL 100.370 m.
Frequently asked questions
Backsight or foresight first?
Backsight is read on the known point; foresight on the new point. New RL = known + BS − FS.
Related tools
- Two-Peg Test Calculator
- Grade & Fall Calculator
- Height of Collimation Worksheet
- Contour Interpolation Calculator
- Rise and Fall Levelling Worksheet
- Vertical Difference Calculator
Explore more in Surveying, Coordinates, GIS & CAD.
Cite this tool
Zerdly. (2026). RL Difference Calculator. Retrieved from https://www.zerdly.com/tools/rl-difference-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.