Calculate rod reading needed at any target grade elevation using the HI (Height of Instrument) method. Works for rotating laser levels, auto levels, and optical instruments.
Grade check faster with a rotating laser level — Spectra Precision LL300N or Topcon RL-H5A.
Shop Express Tools →The Height of Instrument (HI) method is the standard technique for grade control with rotating lasers, auto levels, and optical instruments. Once you establish the HI, you can check or set any elevation in the area without moving the instrument.
Set the laser over or near a known benchmark. Measure the instrument height (height of the beam above the benchmark). HI (height of instrument) = benchmark elevation + instrument height. To check grade at any target point: rod reading = HI − target grade elevation. If the rod reads that value, the bottom of the rod is on grade.
The HI (Height of Instrument) method calculates the elevation of the instrument beam: HI = benchmark elevation + backsight rod reading. Once HI is known, you can find the elevation at any point by: elevation = HI − rod reading at that point. The reverse (finding what rod reading to look for at a known design elevation) is: rod reading = HI − design elevation.
Rod reading needed = HI − design subgrade elevation. Example: HI = 105.50 ft, design subgrade = 100.00 ft → rod reading = 5.50 ft. Place the rod on the subgrade surface — when you read 5.50 ft on the rod (where the laser hits), you are exactly on design grade.
A rotating laser level shoots a horizontal plane (used for site grading, floor leveling, elevation checks). A pipe laser projects a beam along the pipe centerline at a set grade angle — used specifically for pipe installation to maintain constant slope. For grade control on site work, use a rotating laser. For pipe grade, use a pipe laser.
Rotating laser levels (Spectra Precision, Topcon, Leica) are typically accurate to ±1/16 inch at 100 feet (roughly ±3mm). Over longer distances, grade rod reading accuracy depends on the rod receiver quality and the steadiness of the rod. For critical grade control on concrete or utility work, confirm with a digital level or total station backsight.
Height of laser beam above instrument point
Known elevation of benchmark/hub
Design elevation at target point