How to Check and Verify GPS Accuracy on a Job Site

Applies to: Trimble R series, Topcon HiPer series, Leica GS series, Spectra SP80, all RTK GNSS rovers

GPS accuracy on a construction site is not constant — it changes with satellite geometry, base station quality, signal obstructions, and atmospheric conditions. A GPS unit reporting RTK Fixed is not automatically within construction tolerance; it is computing a fixed-ambiguity solution, which is a mathematical confidence level, not a guarantee about accuracy relative to your project control. The only way to know your GPS is performing within tolerance on a given site is to check it against known control. This guide covers the standard field procedures for GPS accuracy verification.

Step 1: Establish a GPS Check Shot Protocol

A check shot is the occupation of a known control point with the rover for the purpose of verifying accuracy — not for establishing a new point. Every crew using GPS for construction layout should check at the start of each day and after any significant change (new base setup, base move, new correction service). On projects with tight tolerances, check every 2 hours.

Identify at least two known control points on or near the site that are not used as the base station occupation point. These check points should be in the project coordinate system with published northing/easting/elevation values. The best check points are NSRS monuments or control set by a licensed surveyor as part of the project control network.

Step 2: Perform the Check Shot

Confirm the rover has RTK Fixed status before occupying the check point — do not attempt a check shot with Float status, as the results are unreliable at the sub-decimeter level. Place the prism pole precisely on the monument top and set the pole height. In the data collector, navigate to the check shot or coordinate comparison routine. The software will display the difference between the rover's observed position and the published coordinates: delta north, delta east, and delta elevation.

Record the residuals. Most field controllers display both component residuals and the combined horizontal and vertical residual. For construction stakeout: horizontal residual under 0.025m and vertical residual under 0.050m are typical project requirements. For structural layout, tighten these to 0.010m horizontal and 0.020m vertical.

Step 3: Monitor Solution Quality Throughout the Day

RTK solution quality is displayed continuously on the field controller. Key indicators to watch:

Solution type: Fixed (best), Float (degraded), Autonomous (unacceptable for layout). Stop all stakeout work if the solution drops to Float for more than 30 seconds.

PDOP (Position Dilution of Precision): A value under 3.0 indicates good satellite geometry. Above 6.0, accuracy is significantly degraded. Check PDOP during work — it increases when satellites are low on the horizon or obscured.

Satellite count: More is better. Under 6 tracked satellites is a risk flag. GPS-only units perform better in the US; multi-constellation (GPS+GLONASS+Galileo+BeiDou) rovers maintain higher satellite counts and better geometry throughout the day.

Step 4: Identify and Respond to Accuracy Failures

If a check shot fails (residuals exceed tolerance), stop stakeout immediately and diagnose. Common causes:

Base station moved or disturbed: Check the base; if the tripod or tribrach shifted, the base needs to be restarted on the control point.

Coordinate system mismatch: Verify the rover is using the same datum, projection, and geoid model as the control.

Antenna height error: Confirm the base antenna height was entered correctly. A 50mm error in antenna height shifts all rover elevations by 50mm.

Check point used as base: Verify you are checking against a point that was not the base occupation point — checking against the base point always shows zero error.

Step 5: Document Accuracy Checks

Record every check shot in the field data collector with the time, control point ID, delta values, and solution quality. This documentation is part of the survey quality record — if a stakeout point is later disputed, check shot records demonstrate the GPS was performing within tolerance when the work was done.