Differential Leveling Calculator

About this calculator

Differential leveling is a surveying technique for transferring a known elevation from a benchmark (BM) to a new point using a leveling instrument and a graduated rod. The surveyor first reads a backsight (BS) — the rod reading taken on the point of known elevation — and adds it to the benchmark’s elevation to get the height of instrument (HI = BM elevation + BS), which is the elevation of the instrument’s horizontal line of sight regardless of where the instrument itself is set up.

The rod is then moved to the new point and a foresight (FS) reading is taken; subtracting it from the height of instrument gives the new point’s elevation: new elevation = HI − FS. The difference between backsight and foresight also tells the surveyor whether the ground rose or fell between the two points (rise if BS exceeds FS, fall if the reverse). Civil engineers, land surveyors, and construction crews use this chained BS/FS procedure — often over many setups in sequence — to run elevation control across a site for grading, drainage design, and construction layout; this calculator applies one setup’s BM, backsight, and foresight to find the new elevation.