GPS machine control brochures cite accuracy numbers under ideal conditions. Field reality is more variable. This guide covers the accuracy you should actually expect from GPS machine control under different site conditions — and what to do when the system is not performing well enough for your specification.
What accuracy should you expect from GPS machine control?
GPS machine control with a good RTK fix typically achieves ±0.05 ft (±15 mm) vertical accuracy at the blade in open conditions. Accuracy degrades to ±0.08 to ±0.12 ft with partial obstruction or weak RTK signal. If the RTK fix is lost entirely, autonomous GPS accuracy is ±1 to ±3 ft — the machine must stop and wait for fix restoration. Always verify finished grade independently of machine control data.
GPS machine control accuracy is not a fixed number — it varies with site conditions, equipment calibration, and signal quality. This table shows what to expect across common field scenarios:
| Condition | Vertical | Horizontal | Suitability |
|---|---|---|---|
| Good RTK, open sky, calibrated mount | ±0.03 to ±0.05 ft | ±0.02 to ±0.03 ft | Finish grade, subgrade |
| Partial obstruction (trees at edge) | ±0.05 to ±0.08 ft | ±0.03 to ±0.05 ft | Rough/finish grade |
| Weak RTK (long baseline or cellular) | ±0.08 to ±0.12 ft | ±0.05 to ±0.08 ft | Rough grade only |
| RTK lost (autonomous GPS only) | ±1.0 to ±3.0 ft | ±1.0 to ±2.0 ft | Stop and wait for fix |
| Antenna mount disturbed (no recal) | ±0.10 to ±0.20 ft | ±0.05 to ±0.10 ft | Unreliable — recalibrate |
Understanding the error sources helps you identify and correct problems before they produce a surface that fails QC inspection:
GPS signals reflecting off cut slope walls, buildings, or equipment before reaching the antenna introduce positioning errors. The receiver cannot easily distinguish direct signal from reflected signal. Multipath errors can be 0.05-0.15 ft in severe cases. On a site with high cut walls on two sides of a cut-and-fill corridor, multipath can be significant enough to require switching to a total station or robotic instrument for the final trim pass.
The blade offset — the geometric relationship between the GPS antenna position and the blade cutting edge — is determined by a calibration procedure. If the antenna mount is struck by debris, shifted by vibration, or not mounted identically after removal and reinstallation, the calibration is wrong. The system will not alert the operator; it will simply grade to the wrong elevation. Check the blade offset calibration with a check shot at a known benchmark at the start of each shift.
PDOP (Position Dilution of Precision) measures satellite geometry quality. A PDOP below 2.0 is excellent; above 4.0, accuracy degrades significantly. PDOP changes through the day as satellites move. Most machine control systems display PDOP on the operator screen. If PDOP exceeds 3.0, consider pausing precision work until geometry improves. GPS planning software (available free from most receiver manufacturers) can predict PDOP windows for your site coordinates and date.
If the base station is set up on an unverified point, or if the base station is bumped or shifted during the work day, all machine control accuracy references from that base are wrong — and they may be consistently wrong in one direction. This is why every base station setup must be verified against a second known control point before the work day begins.
Whether GPS machine control is accurate enough depends on your project's grade tolerance requirement:
Verify machine control performance every day before trusting it with production work:
Use Sitemark to capture rover verification shots, compare to design, and generate a QC report that documents the finished grade for owner acceptance. This independent verification protects you regardless of what the machine control system logs.
In good conditions with strong RTK fix: ±0.03 to ±0.05 ft vertical. With partial obstructions or weak signal: ±0.05 to ±0.12 ft. Without RTK fix (autonomous GPS): ±1 to ±3 ft — stop grading until RTK is restored.
Main causes: multipath interference from walls or trees, antenna mount calibration drift after the machine is transported, high PDOP from poor satellite geometry, base station on an unverified point, and RTK signal loss or downgrade to Float solution.
For a ±0.10 ft subgrade specification, machine control at ±0.10 ft is marginal — you will have areas that require re-grading after the QC survey. For a ±0.05 ft subgrade specification, GPS machine control is adequate in good conditions but requires QC verification to confirm. For concrete subgrade at ±0.03 ft, use laser machine control for the final trim pass.
Sitemark captures rover verification data, compares to design, flags out-of-tolerance areas, and generates QC reports for owner acceptance. Start free.
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