Ground Control Points
Surveyed reference targets placed in the scene before photogrammetry or drone survey, used to scale and georeference the output to real-world coordinates.
Ground Control Points (GCPs) are targets — typically printed markers or physical targets with known real-world coordinates — that are placed in the survey area before photography begins. They appear in multiple overlapping photographs, and their known positions are used by photogrammetry software to calculate the absolute scale and geographic position of the resulting 3D model.
Without GCPs, a photogrammetric model has accurate relative geometry (shapes and proportions are correct) but no absolute scale or georeferencing — the model floats in its own arbitrary coordinate system. For heritage documentation where real-world dimensions and positions are required, GCPs are essential.
GCP positions are measured using a total station or GNSS receiver (GPS). For precise georeferencing, a minimum of four GCPs is typically recommended, distributed around the edges and centre of the survey area. More GCPs improve the accuracy of the georeferencing, particularly for large or complex surveys.