Ground control points (GCPs) explained
What are ground control points, why are they crucial and how many do you need?
Ground Control Points (GCPs) are clearly visible points on the ground whose exact coordinates we measure classically. They anchor the drone model in the real world.
Why GCPs are important
Without anchoring, a photogrammetric model 'floats': the shape is correct, but not necessarily the absolute position, scale and orientation. GCPs link the model to the coordinate system and guarantee the absolute accuracy.
How many do you need?
Rule of thumb: at least 5 well-spread points for a small project, more for larger or elongated zones. We use some as checkpoints to check the accuracy independently.
With RTK/PPK the number of GCPs can decrease, but a few control points remain advisable to verify the result.