Quick Answer
How do you perform a collimation check on a digital level?
A collimation check (two-peg test) involves shooting a staff from near and far positions on two pegs set the same distance apart. Compare the height difference measured from near vs. far. Any discrepancy indicates a line-of-sight error. Most digital levels let you enter the computed correction so the instrument compensates automatically. Check collimation whenever the instrument is transported, dropped, or producing inconsistent results.
How to Perform a Collimation Check on a Digital Level
Applies to: Leica Sprinter, Topcon DL-503, Sokkia SDL1X, Trimble DiNi, Spectra Precision LL Series digital levels
A digital level is trusted to measure elevation differences to 0.1mm or better on precise leveling runs. That trust depends on the instrument's line of sight being truly horizontal — which drifts over time from transport vibration, temperature changes, and general field use. The collimation check, often called a two-peg test, takes about 20 minutes and tells you exactly how much line-of-sight error your instrument has. Most digital levels allow you to enter the computed correction and compensate automatically.
Step 1: Set Out Two Pegs at Known Distance
Drive two sturdy pegs (or use two stable points on a flat surface) separated by 40-60 meters. Mark each peg top clearly so you can consistently read the same point. The instrument will be set up at three positions during this test, so choose a setup location in the middle and one at one end. Ensure the ground is stable — soft ground that allows tripod settling will contaminate the collimation measurement.
Step 2: First Setup — Middle Position
Set up the digital level exactly halfway between the two pegs (20-30m from each). Level carefully using the instrument's circular bubble and electronic compensator. Shoot Peg A and record the staff reading, then swing and shoot Peg B. The height difference between A and B from the midpoint setup (HD_mid) is your reference value — because the instrument is equidistant from both staves, any line-of-sight error cancels out. This is your "true" height difference.
Step 3: Second Setup — Near One Peg
Move the instrument to a position 2-3 meters beyond Peg A (so Peg A is near and Peg B is far — approximately 40-60m away). Level carefully again. Shoot Peg A at the short distance (record reading RA_near) and shoot Peg B at the long distance (record reading RB_far). Compute the height difference from this position: HD_far = RA_near - RB_far.
Step 4: Calculate the Collimation Error
Compare the two height differences:
Collimation error = HD_far - HD_mid
If this value is zero or very small (under 0.5mm over 50m), the instrument is within acceptable tolerance. Most survey-grade digital levels specify collimation tolerance as ±0.5mm per 50m of sight. If the error exceeds this, correction is needed before the instrument is used for precise work.
Convert to an angular error if needed: divide the error in mm by the backsight/foresight distance in meters to get the error in radians, or express as mm/km for comparison to instrument specifications.
Step 5: Apply the Correction
Most modern digital levels (Leica Sprinter, Topcon DL-503, Sokkia SDL1X) have a built-in collimation correction routine accessible through the instrument menu. Enter the computed correction value and the instrument will apply an automatic offset to all subsequent readings to compensate for the line-of-sight error.
If the instrument does not have an auto-correction feature, the line of sight can be mechanically adjusted using the collimation adjustment screw (consult the instrument manual for location). Make the adjustment in small increments and repeat the test until the error is within tolerance.
Step 6: Confirm with a Repeat Check
After applying the correction, repeat the two-position test to confirm the collimation error is now within tolerance. Document the pre-correction error, correction applied, and post-correction error in the job calibration record. For Level 1 or precise leveling work, perform the collimation check at the start and end of each day's leveling.
Frequently Asked Questions
How often should a digital level collimation check be performed?
For general construction leveling, check collimation monthly or when the instrument has been transported over rough terrain or dropped. For precise leveling (benchmark networks, settlement monitoring), check at the start of each leveling session. NSPS and FGDC specifications for vertical control surveys specify collimation checks at least daily.
What is an acceptable collimation error for a digital level?
Most manufacturers specify the collimation tolerance as ±0.5mm per 50m (or equivalently ±10 seconds of arc). For general construction work, errors up to 1mm/50m are often tolerable. For benchmark surveys or precise work, correct any error exceeding 0.3mm/50m.
Can a digital level automatically compensate for collimation error?
Yes — most modern digital levels (Leica Sprinter 150/250, Topcon DL series, Sokkia SDL series) have a collimation correction mode where you enter the computed error and the instrument applies an automatic offset to level readings. Check your instrument's user manual for the specific menu path.
What is the difference between a collimation check and a two-peg test?
They are the same test by different names. The two-peg test (or collimation check) uses two pegs at a known distance to isolate the instrument's line-of-sight error by comparing height differences measured from equal and unequal distances. Both names refer to the identical procedure.
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