A ground control point (GCP) is a physical marker used to indicate a specific location on the ground. They are often placed at corners of a property or along straight lines, such as roadways. GCPs can be made from a variety of materials, such as metal pipes, rebar, or even survey flags. When using photogrammetry to survey an area, GCPs are placed at strategic locations in order to help the software calculate accurate measurements.
GCPs are an important part of any surveying project that uses photogrammetry with inaccurate georeferenced imagery. Without GCPs or accurate camera external parameters, it would be very difficult to get accurate measurements. By placing GCPs at key locations, you can be sure that your measurements are as accurate as possible.
A GCP can be anything that is easily identifiable and referenced in the photogrammetry project. A precise point on the GCP should be easily and repeatably selectable from within photogrammetry software.
An accurate coordinate of the reference point in the GCP needs to be obtained in order for it to be useful, this can be done with RTK GPS or with PPK workflows.
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