The two-peg test is the standard field calibration check for rotating laser levels. Set up at the midpoint between two pegs 200 ft apart; take rod readings at both pegs to find the true elevation difference. Move close to one peg and repeat. If the two HI values give an inconsistent elevation difference, the compensator is out of adjustment and the instrument should be sent for factory calibration.
Why Laser Level Calibration Matters
A rotating laser that is not truly horizontal produces systematic errors in every elevation shot. If the beam tilts 0.01 ft per 100 ft, a grade check at 400 ft has a 0.04-ft built-in error — enough to fail a ±0.05-ft pavement subgrade tolerance, with all errors pointing the same direction. The problem is invisible until you check the instrument against a known benchmark or run the two-peg test.
The Two-Peg Test
The two-peg test works because, at the midpoint between two pegs, compensator errors are equal and opposite and cancel out. The off-center setup reveals the error.
Procedure
- Drive two stable pegs (or use existing hubs) 200 ft apart.
- Midpoint setup: Set the laser exactly 100 ft from each peg. Take rod readings at both pegs: R1 (Peg 1) and R2 (Peg 2). True elevation difference: ΔH = R1 − R2.
- Offset setup: Move the laser close to Peg 1 (~5–10 ft away). Take rod readings at both pegs: R3 (Peg 1) and R4 (Peg 2). Apparent ΔH = R3 − R4.
- Calculate error: Error = Apparent ΔH − True ΔH over 200 ft. If |Error| > 0.012 ft (0.006 ft/100 ft threshold), the instrument needs calibration.
Offset: R3 = 4.67, R4 = 4.77 → Apparent ΔH = −0.10 ft
Error = −0.10 − (−0.13) = +0.03 ft over 200 ft = 0.015 ft/100 ft
Exceeds threshold — instrument needs calibration
When to Check Your Laser
- Start of every project — verify before depending on it.
- After any significant impact (dropped instrument, knocked tripod).
- After transport on rough roads.
- Any time unexplained grade check failures occur.
- Minimum: every 3 months for actively-used instruments.
Factory vs Field Adjustment
Most rotating laser levels have no user-adjustable compensator. The two-peg test tells you whether the instrument is in spec; if it is not, send it to an authorized service center. Field mechanical adjustments typically void the calibration certification and make things worse.
See also: Reading a Laser Level: Rod Setup and Grade Calculations and How to Set Grade with a Rotating Laser Level.