Quick Answer
How do you perform a two-peg test on an optical level?
Set up the level midway between two rods 100-200 feet apart, read both rods, and calculate the true height difference. Then move the level near one rod and read both again. Compare the measured height difference from the close position to the true value. Any discrepancy equals the collimation error. Adjust the reticle until readings match.
How to Perform a Two-Peg Test on an Optical Level
Applies to: Topcon AT-B4, Leica NA724, Spectra Precision AL24, Sokkia B40
The two-peg test is the standard field method for checking whether an optical level's line of sight is truly horizontal. A level with collimation error — where the line of sight tilts slightly up or down — produces readings that appear valid but contain a systematic elevation error that grows with sight distance. A crew running differential leveling with an unchecked instrument can build significant elevation error into a benchmark network without knowing it. The two-peg test catches this, quantifies the error, and — if the instrument has user-accessible adjustments — allows field correction.
Understanding Collimation Error
When an optical level is perfectly adjusted, the line of sight through the telescope is exactly horizontal when the instrument bubble is centered. Collimation error means the line of sight is actually tilted slightly upward or downward relative to true horizontal. The error is proportional to distance — at 100 feet, a small tilt produces a small error; at 500 feet, the same tilt produces five times the error. This is why short, balanced sight distances are taught in precision leveling: balanced distances cancel collimation error even in an instrument that's out of adjustment.
The two-peg test works by comparing rod readings from a position where sight distances are equal (where collimation error cancels out) to readings from a position where sight distances are unequal. Any difference reveals the collimation error.
Step 1: Set Up the Midpoint Position
Drive two stakes (the "pegs") into level ground approximately 200 feet apart. Label them Peg A and Peg B. Set up the optical level exactly midway between them — at 100 feet from each peg. Level the instrument carefully until the circular bubble is centered and stays centered as you rotate. Read the rod held vertically on Peg A. Record this reading (for example: 4.892'). Read the rod on Peg B. Record this reading (for example: 5.137').
The difference from the midpoint position is the true height difference between the two pegs, because the collimation error is equal and opposite at the two rods and cancels out. True height difference = Rod A - Rod B = 4.892 - 5.137 = -0.245' (Peg A is 0.245' higher than Peg B). Write this down — it is your reference value.
Step 2: Move the Level Near Peg A
Move the level and set it up approximately 5-10 feet from Peg A (still on the line between the two pegs). Read the rod at Peg A from this very short distance — call this reading "a2" (for example: 4.910'). Read the rod at Peg B from the long distance (~195 feet away) — call this reading "b2" (for example: 5.170').
Calculate the measured height difference from this position: a2 - b2 = 4.910 - 5.170 = -0.260'. Compare to the true height difference: -0.245'. The discrepancy is -0.260 - (-0.245) = -0.015'. This means the instrument's line of sight reads 0.015' too low at 195 feet — the collimation error is 0.015' per 195' = 0.000077' per foot, or about 0.008' per 100 feet.
Step 3: Evaluate the Error
Standard acceptance criteria for optical level collimation error: less than 0.005' per 100 feet (approximately 1.5mm per 30m) for construction work. Less than 0.003' per 100 feet for precision leveling. If your calculated error exceeds the acceptable threshold, the instrument needs adjustment.
On the Topcon AT-B4 and similar automatic levels, collimation adjustment is done by raising or lowering the crosshair reticle using the reticle adjustment screws (usually accessible through a cover plate on the eyepiece end). The procedure: with the level still at the Peg A near position, calculate what the correct rod reading at Peg B should be (true height difference + a2 reading). Adjust the reticle until the telescope reads this calculated value on the Peg B rod. Repeat the full two-peg test to confirm the adjustment.
Step 4: Document the Results
Record the test results: date, instrument model and serial number, collimation error calculated, whether adjustment was performed, and the post-adjustment error. Instruments in daily use should be tested at least monthly. An instrument that consistently fails or whose collimation drifts rapidly needs factory service.
Frequently Asked Questions
How often should I run the two-peg test?
Run the two-peg test monthly for instruments in daily construction use, and after any drop, shipment, or rough handling. Also run it before any precision leveling job where benchmark accuracy matters.
Can I run the two-peg test with a digital level?
Yes — the procedure is identical. Read the bar-coded rod from the midpoint and from the near position. Most digital levels (Leica Sprinter, Topcon DL-series) display readings directly and some have a built-in collimation check mode that automates the calculation.
What is an acceptable collimation error for construction leveling?
For general construction work, collimation error under 0.005' per 100 feet is acceptable, especially when balancing sight distances in the field. For benchmark leveling or work requiring high precision, target under 0.003' per 100 feet.
Why does the two-peg test use equal sight distances for the first position?
Equal sight distances at the midpoint cause the collimation error to affect both rod readings equally, so the error cancels when calculating the height difference. This gives you the true height difference regardless of the instrument's collimation state.
Log two-peg test results, collimation error values, and adjustment dates for every optical level in your fleet with Gradelog's Equipment Registry. Free to start at gradelog.com.
