Zerdly

Two-Peg Test Calculator

Computes level collimation error from two-peg test readings for instrument calibration.

100% FreeRuns in your browserIntermediate
How to use this calculator

Inputs

Collimation error from a midpoint and a near setup, using the actual sight distances.

Midpoint setup
Near setup
More options
Reporting and your limit

Baseline for the mm-per-distance figure; 30 m when blank.

mm

From your own procedure or the manufacturer — no default limit is assumed.

Enter the values and press Calculate.

Or load an example to see a worked result.

Your two-peg test
Schematic
Your two-peg testSchematic midpoint and near setups between staffs A and B.ABmAmBSetup 1: midway — the error cancelsABnAnBSetup 2: near A — the error does not canceldAdB
Calculate to label both setups with your readings and sight distances.

How the method works

Two setups, one collimation error
Illustrative method
Two setups, one collimation errorStaffs A and B read from an instrument set midway, then from an instrument set near A, with the line of sight tilted (exaggerated) and the near and far sight distances dimensioned.ABmA 1.500mB 1.000Setup 1: midway — the error cancelsABnA 1.200nB 0.670Setup 2: near A — the error does not canceldA 5 mdB 35 m
Reading model r = HI − H + e × sight distance. Midway, the error is equal on both staffs and cancels: true ΔH = 1.5 − 1.0 = 0.5 m. Near A with sights of 5 m and 35 m, 1.2 − 0.67 = 0.53 m, so e = (0.53 − 0.5) ÷ (5 − 35) = −0.001 m/m = −30 mm per 30 m. Use the actual sight-distance difference. Tilt exaggerated.

Fixed example values for illustration — not your result.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Set up midway between pegs A and B and read both staffs.
  2. Set up close to one peg, read both staffs again, and enter the actual sight distance to each.
  3. If the first setup was not exactly midway, choose unequal midpoint sights and enter those distances too. Read the error per metre and per your baseline.
  4. Under More options set the reporting baseline, your own acceptance limit and the instrument name; the calibration record table exports as CSV and the Print report gives a calibration sheet.

How it works

Each reading is modelled as HI − H + e × d, where e is the collimation error per metre of sight (positive e raises readings with distance).

From the midpoint the error cancels, so mA − mB is the true difference. At the near setup nA − nB = true difference + e(dA − dB), so e = ((nA − nB) − (mA − mB)) ÷ (dA − dB). Unequal midpoint sights subtract (dMA − dMB) from that denominator.

Equal near-setup distances cancel the error and cannot measure it, so the tool refuses them. A corrected reading is reading − e × d.

Worked example

Midpoint 1.5 / 1.0, near 1.2 / 0.67 at 5 m and 35 m. e = (0.53 − 0.5) ÷ (5 − 35) = −0.001 m/m, or −30 mm per 30 m. The corrected near readings 1.205 and 0.705 differ by the true 0.5 m.

Common mistakes

  • Assuming the sight distances instead of measuring them.
  • Treating a midpoint setup as exact when it was paced out roughly.
  • Adjusting the instrument from these numbers instead of following the maker's procedure.

Frequently asked questions

How much collimation error is acceptable?

Use your own or your organisation's limit (often stated per 30 m or per sight length). Adjust or service the instrument following the manufacturer's procedure when it is exceeded.

Related tools

Explore more in Surveying, Coordinates, GIS & CAD.

Cite this tool

Zerdly. (2026). Two-Peg Test Calculator. Retrieved from https://www.zerdly.com/tools/two-peg-test-calculator/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.