A ground control point (GCP) is a feature that you can clearly identify in the raw image for which you have a known ground coordinate. You can obtain the coordinates for the points from many sources such as geocoded images or vectors. Although the media, formats, and methods used to collect the coordinates are different depending on the source, the idea is the same. You have to match a point in the raw image to a set of coordinates. Since some sources of ground control only offer dispersed points, it may be more efficient to select a point in the source first and then seeking it in the raw image. For example, a vector file may have a limited number of features available as ground control compared with a geocoded image.
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