Documentation | Help File | Glossary | GCP (Ground Control Point)

A GCP is an exact global point, of known geographical coordinates, that can be precisely identified on an aerial or satellite image or map. Each GCP is defined with an x,y coordinate set. Dependent on the coordinate system of the x,y coordinate set, the x,y coordinates often represent longitude and latitude decimal/degrees. Each GCP is precisely assigned to an aerial image, scanned map, or vector map layer at locations dispersed relatively evenly across the coverage.The GCP coordinates and placements on the coverage are used by the rectification calculation to transform the entire coverage to match a grid that reflects a specific geographic coordinate space.

The coordinates of the GCPs may reflect any Editor supported coordinate system, which includes nearly 3,000 pre-defined coordinate systems and any user-defined coordinate system.

The accurate placement of GCP points to an aerial image or vector coverage is crucial to achieving accurate rectification results (low RMS error). The best GCPs can be obtained from geographic survey data, but quality GCPs can also be obtained from high-resolution topographical maps of the subject terrain, GPS readings collected on the ground, or already georeferenced aerial images, raster maps, or vector maps. The GCP coordinates can be entered via the keyboard or by using the mouse to lift the coordinates from a matching point on an already georeferenced image or vector coverage opened in the Rectify reference window.