Glossary · Drones
Ground Control Points (GCPs)
Surveyed reference markers placed on the ground during a drone flight. The single largest factor in drone-based silage volume accuracy.
Ground Control Points (GCPs) are surveyed reference markers placed around a pile during a drone flight. They are the single largest factor in drone-based silage volume accuracy.
A drone alone produces a 3D model from photogrammetry, but the model’s coordinate system is approximate — drift in onboard GPS is enough to introduce meaningful error in volume measurements. GCPs anchor the model to known coordinates, eliminating that drift.
A typical setup:
- 5–8 GCPs per pile, placed at corners and midpoints of the perimeter.
- Surveyed positions accurate to centimeter level, typically captured with an RTK-GPS rover.
- Visible from above — bright targets that the photogrammetry pipeline can identify automatically.
The accuracy difference is meaningful:
- Drone alone (no GCPs): ±10–20% volumetric error, drifting between flights.
- Drone with GCPs and accurate positioning: ±2–5% error, repeatable across flights.
For a dairy that wants to track shrink over time, GCP-anchored measurements are the only way the trend is real signal rather than noise. See Accurate Forage Inventory: Will Any Drone Work? for the full discussion.