Ground Control Point (GCP)
Definition
A physical marker in the scene whose three-dimensional coordinates have been measured independently by total station or GPS. Assigning those coordinates to the marker's location in the photogrammetric model scales and georeferences the model.
- Also known as
- GCP
- Measured by
- Total station or GPS, independently
- Function
- Scales and georeferences a photogrammetric model
- Use
- Close-range photogrammetry and 3D scene recording
Common questions
Why can't a photogrammetric model be trusted for measurement without ground control points?+
Photogrammetry software can reconstruct the correct shape and relative proportions of a scene from overlapping photographs alone, but without at least a few points of known real-world coordinates, the resulting model has no fixed scale or orientation, so distances measured in it could be wrong by an unknown factor.
How many ground control points does a typical forensic scene model need?+
There is no single fixed number, but practitioners generally use several GCPs spread across the scene rather than the bare minimum, so the software can cross-check consistency and reduce georeferencing error rather than relying on any single point that might itself be mismeasured.
Related terms
- Dense Point Cloud
- A set of millions of three-dimensional coordinate points, each with an RGB colour derived from the photographs, that together represent the surface...
- Orthophoto
- A geometrically corrected aerial or close-range photograph from which the distortions caused by camera tilt and terrain relief have been removed, so...
- Reprojection Error
- The distance in pixels between where the photogrammetry software predicts a feature point should appear in each photograph (based on its estimated...
- Structure-from-Motion (SfM)
- A photogrammetric technique in which overlapping images from a drone or camera, processed by software, generate a three-dimensional point cloud and textured...
- Terrestrial Laser Scanning (TLS)
- A ground-based 3D LIDAR survey instrument that captures millions of range measurements per scan to produce a millimetre-accurate point cloud of the...