Verify machine guidance accuracy by shooting a total station check on the blade position: set up the total station on control, aim at the prism on the blade mast, and compare the measured blade tip elevation to the design surface elevation at that location. Do this at the start of each work day and after any GPS signal outage or base station restart. The machine’s displayed cut/fill is only as accurate as the calibration.
Why Independent Verification Is Necessary
Machine guidance systems display the blade position relative to the design surface, but the display accuracy depends on: (1) the base station being accurately positioned over a known point, (2) the GPS receiver on the machine having a Fixed RTK solution, (3) the blade calibration being current, and (4) the correct design surface being loaded. Any of these can fail silently. An independent check with a total station or level catches these failures before the crew grades 50,000 sq ft to the wrong elevation.
Total Station Blade Check Procedure
- Set up the total station on a control point and orient with a backsight.
- Direct the machine operator to position the blade at the design surface at a specific, checkable point (a stake location or a known grade point).
- Mount the prism pole or use the blade mast prism (if installed) and shoot the prism position with the total station.
- Compare the total station-measured elevation at the blade tip to the design elevation from the plans at that location.
- If the difference is within tolerance (±0.05 ft typically), the machine guidance is accurate. If not, investigate before continuing.
Level and Rod Check (Simpler Field Method)
A faster alternative for routine daily checks:
- Set up the automatic level on a benchmark. Calculate HI.
- Hold the rod on the blade tip when the machine is on a known grade point.
- Calculate the blade tip elevation: blade elevation = HI − rod reading.
- Compare to design surface elevation at that point.
When Machine Guidance Accuracy Degrades
- GPS signal obstruction: The solution drops to Float or loses fix entirely. The display may still show a value. Never accept Float-solution machine control accuracy for finish grading.
- Base station moved or radio connection lost: The machine is now running without corrections — accuracy degrades to ~1–3 meters. Verify immediately after any base station issue.
- Blade calibration drift: Heavy use, blade impact, or machine service can shift the calibration. Re-calibrate after any significant blade impact event.
See also: Machine Guidance Grade Control: Overview and Total Station Setup and Backsight Procedure.