Ground control points

Ground Control Points (GCPs) are points that can be clearly identified in the image and on the target map. The target map could be a topographic map or another image that has been transformed beforehand to the desired map projection system. The operator then needs to identify corresponding points on both images. The image and map scale determine which points are suitable. Typical examples of suitable points are road crossings, crossings of waterways and salient morphological structures. Another possibility is to identify points in the image and to measure the coordinates of these points in the field, for example by GPS, and then transform those to map coordinates. It is important to note that it can be quite difficult to identify good GCPs in an image, especially in lower-resolution space-borne images.