Calculate Height of Instrument (HI) and point elevations from benchmark and rod readings. Supports single-setup leveling and multi-point differential leveling with turning points and closure check.
Need a builder's level, laser level, or digital rod for field leveling? Shop survey equipment.
Shop Express Tools →HI (Height of Instrument) = Benchmark Elevation + Backsight reading. Point Elevation = HI − Foresight reading. A higher foresight reading means the point is lower than the instrument. A lower foresight means the point is higher. Closure = Σ Backsights − Σ Foresights.
Differential leveling is the backbone of construction survey control. Every grade stake, form elevation, and finished surface check flows back to this calculation. The power of the method is that errors in individual readings don’t propagate forward — each point is calculated independently from the HI. Only the HI itself depends on the backsight accuracy. This is why taking a careful backsight on a stable, known point is critical: a bad backsight ruins all subsequent elevations from that setup.
| Survey Class | Max Closure Error | Typical Use |
|---|---|---|
| Construction grade check | ±0.05–0.10 ft | Rough grade, earthwork, general layout |
| Engineering / third order | 0.05√K ft (K=miles) | Construction control benchmarks, roads |
| Second order class II | 0.035√K ft | City benchmarks, bridge control |
| Second order class I | 0.025√K ft | State plane control, major structures |
| First order | 0.017√K ft | National geodetic network |
Log survey data and elevations with Sitemark →
Record rod readings, benchmark data, and as-built elevations with GPS location and photos. Build a permanent field record for every grade check and survey shot.
Start free trialHeight of Instrument (HI) = Benchmark Elevation + Backsight (BS). Foresight Elevation = HI − Foresight (FS). In other words: look back to a known point to establish your instrument height, then look forward to find the unknown elevation.
A backsight (BS) is a rod reading taken on a known or established elevation point. A foresight (FS) is a rod reading taken on an unknown point whose elevation you want to find. Backsights add to the benchmark elevation; foresights subtract from the HI.
Differential leveling is the process of running a series of setups between two points, using turning points (TPs) to extend the level circuit. At each setup, you take a foresight on the last TP and a backsight on the next. This calculator handles multi-point runs with a closure check.
Closure is the sum of backsights minus the sum of foresights. For a closed loop or a run with a known endpoint, closure tells you the accumulated error in the survey. A perfect loop has zero closure; acceptable limits depend on survey precision requirements (e.g., third-order leveling ≤ 0.05√K feet, where K = distance in miles).
A builder's level or automatic level is set up on a tripod, and a surveying rod (Philadelphia rod, Lenker rod, or digital grade rod) is held on the point. The instrument operator reads the horizontal crosshair on the rod through the scope. Digital levels use bar-coded rods and give automatic readings.
HI = BM Elevation + BS | New Elevation = HI − FS