Ground Control Points (GCPs) are recognizable features in your dataset that have an assigned coordinate location. These features can be purpose-made panels, spray painted shapes, corners from sidewalk intersections, or even the center of trees. The coordinates always include X and Y values and most often it includes Z values too. These locations can be determined through ground GPS measurements (survey equipment or hand-held GPS) or through base map imagery. Size of the area of interest, spatial resolution, terrain changes, and other factors all can influence the necessary quantity and distribution of ground control.