Machine control accuracy is only as good as the last calibration. A GPS antenna offset that is off by 0.05 ft introduces a constant error into every cut and fill the machine makes — and that error compounds across an entire project. Documenting calibration is both a QC requirement and a protection against disputes about grade accuracy.
What calibration documentation is required for GPS machine control systems?
GPS machine control calibration documentation requires: (1) initial calibration record when the system is installed or moved to a new project — antenna offset measurements, system configuration settings, and a before/after verification check against a known elevation; (2) daily pre-shift GPS check showing the rover or machine GPS reads within ±0.03 ft of a known control point; (3) blade offset verification — performed any time the blade is changed or the mast is repositioned, recording the measured antenna-to-blade-tip distance in each axis; (4) in-field spot check records — at least once per shift, a GPS rover shot on recently machine-graded material compared to the design surface, with the deviation recorded; and (5) any re-calibration records triggered by spot check failures or after a satellite outage or system restart. These records constitute the machine control QC documentation for DOT and government projects.
The machine control system knows the position of the GPS antenna precisely. What it infers — not directly measures — is the position of the blade cutting edge. The system calculates the blade position from the antenna position using a set of offset measurements entered during calibration:
Any error in these offset measurements produces a systematic error in every grade the machine cuts. A 0.05-ft error in the antenna height offset means every cut is 0.05 ft too high or too low — across 10 acres of grading, this can result in thousands of cubic yards of misgrade.
Machine control systems require recalibration in the following circumstances:
| Trigger Event | Calibration Action Required |
|---|---|
| Blade change or replacement | Full blade offset calibration — blade geometry may differ |
| GPS mast repositioned or repaired | Antenna offset re-measurement and GPS check |
| Spot check deviation greater than ±0.05 ft | Identify cause — GPS check, offset re-verification, design surface check |
| System transferred to different machine | Full calibration for new machine geometry |
| After extended satellite outage (>10 minutes) | GPS accuracy check before resuming grading |
| Any significant collision or impact to mast | Full calibration — impact may have shifted antenna position |
| Move to new project | Confirm design surface and coordinate system loaded correctly; GPS check on local control |
The machine control calibration log documents every calibration and verification event for a machine on the project. The log should be maintained by the machine operator or foreman and submitted to the QC manager at the end of each week. Required log fields:
When a grade dispute arises — an owner claims the machine-graded surface is out of tolerance, or an inspector fails a grade check — the calibration log is the first document requested. A complete calibration log demonstrates:
Without a calibration log, a grade dispute devolves into the operator's word against the inspector's survey. With a calibration log, the burden shifts: if the log shows the machine was verified on grade that day, the claim that the machine produced out-of-tolerance work requires additional evidence.
Sitemark connects machine control calibration logs with GPS spot check records and finish grade verification surveys — giving you a complete, timestamped machine control QC record. Start free.
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