3D machine control (GNSS/GPS automated grading) is flawless on flat, prepared subgrade. However, contractors are discovering a severe limitation when using it for rough grading on loose, uncompacted fill or rock. The system fails due to "surface uncertainty."
Machine control relies on sensors-total stations, or even slope sensors on the blade or bucket-to determine the machine's position relative to the digital design. If the machine is working on rough, rocky ground, the wheels or tracks are constantly bouncing. The GNSS receiver on the roof is moving up and down at a high frequency. The software applies an algorithm to smooth this data, assuming the bumps are just suspension travel.
But if the ground is too rough, the "noise" exceeds the software's filter capability. The blade thinks it is flying through the air or burying itself, even if it's actually on the ground. The machine begins to auto-correct violently, trying to chase a phantom elevation. The result is a "washboard" surface-severe waves in the earth that are physically impossible to fix without resetting the system and switching to manual mode. In these conditions, the machine control becomes a liability, and the operator must manually rough-grade the site smooth enough for the automatic system to take over.
