Calculate point elevation from a benchmark using the height of instrument (HI) method. Also calculates required rod reading to set a point at a target elevation. Two modes: calculate elevation from rod readings, or calculate required rod for a target elevation.
Automatic levels, digital levels, and grade rods for benchmark elevation transfer surveys. Topcon, Leica, and Spectra equipment available.
Shop Express Tools →The height of instrument (HI) method is the fundamental technique for transferring elevations in the field. It is used every time a grade check, pad elevation survey, pipe invert survey, or foundation layout requires elevations referenced to a known benchmark.
Benchmark BM-1 elevation = 452.78 ft. Backsight rod reading on BM-1 = 4.82 ft. HI = 452.78 + 4.82 = 457.60 ft. Foresight rod reading at point A = 6.14 ft. Point A elevation = 457.60 - 6.14 = 451.46 ft.
To set a grade stake at design elevation 452.10 ft from the same setup: Required rod reading = 457.60 - 452.10 = 5.50 ft. Hold the rod at the stake location and raise or lower the stake until the rod reads 5.50 ft. At that point the top of the stake is at elevation 452.10 ft.
A well-leveled automatic level with a precise rod achieves ±0.01 ft per setup under good conditions. Error sources include: instrument not level (check bubble before every reading), rod not held plumb (use a rod level or wave the rod slightly and read the minimum), rod not fully extended (check extension locks), refraction on hot days (take readings in morning when possible), and wind vibration on the rod (wait for calm conditions on windy days).
For critical surveys, run a level loop back to the starting benchmark and calculate the closure error. Allowable closure error for third-order leveling is typically 0.05 ft × √(distance in miles). For construction surveys, a closure within 0.02–0.05 ft per mile is generally acceptable. Distribute the closure error through the loop by distance (Bowditch method) if needed.
The height of instrument (HI) method is a standard differential leveling technique. The instrument is set up near the benchmark, a rod is held on the benchmark and read (backsight), and the HI is calculated as benchmark elevation + backsight rod reading. The HI is the elevation of the instrument's line of sight. To find the elevation of any other point, hold the rod there (foresight) and subtract the foresight rod reading from HI. Point elevation = HI - foresight.
Set up your level between the benchmark and the target point. Read the rod on the benchmark (backsight) and calculate HI = benchmark elevation + backsight. Move the rod to the target point and read the rod (foresight). Target elevation = HI - foresight. For long distances or complex terrain where you cannot see both points from one setup, use turning points to carry the elevation through a series of instrument setups (differential leveling run).
A required rod reading (also called a grade rod or cut rod) is the rod reading that corresponds to a specific target elevation. When a grade stake is being set, the rod person holds the rod while the instrument person calls out the required rod reading. The rod person moves the stake up or down until the rod reads the required value — at that point, the top of the stake is at the target elevation. Required rod reading = HI - target elevation.
A turning point (TP) is an intermediate point used to carry an elevation from one instrument setup to the next when the benchmark and target point are too far apart or obstructed. The procedure: set up the level, take a backsight on the known benchmark to calculate HI, move to a new position, take a foresight on the turning point to calculate its elevation, then take a new backsight on the turning point from the new setup. The elevation error accumulates with each turning point, so minimize the number of TPs when precision is critical.
A permanent benchmark (BM) is a permanent monument established by a government agency or licensed surveyor with a precisely known elevation in a standard datum (NGVD 29 or NAVD 88). A temporary benchmark (TBM) is a stable, semi-permanent point established for a specific project — typically a chiseled square on concrete, a nail in a tree root, or a stake with a hub nail. TBMs are referenced to a permanent BM when the project begins. Always document TBM elevation, location description, and the permanent BM it was set from.
Calculate point elevation from a benchmark using HI method: HI = Benchmark + Backsight. Point Elev = HI - Foresight.
Known benchmark or TBM elevation
Rod reading on benchmark
Rod reading at unknown point