Aerial survey data must be referenced to ground control points in order to maximize the absolute accuracy achievable for the aerial data. GCPs increase the absolute accuracy of a project, placing the model at the exact position on the Earth. They reduce the shift due to GPS from meters to centimeters. This is achieved by ground crews establishing photo ground control within the project area. Targets are placed over ground control so that the location of the point is easily identified on the imagery. The field measurement of the horizontal and vertical elevation (X, Y and Z) of the control points will be used in the downstream processes of photogrammetry and point cloud calibration to register the data sets. Elevations, (Z), must be provided at surface grade.
At CloudD8ta, we use ground control points to check the quality of our work. While measuring our GCPs, we also lay down check points. These checkpoints are then used in our error analysis to determine the quality of our work. Some photogrammetrists will tell you that they can use the same GCPs from the processing workflow as checkpoints but this is false.
The accuracy of the GCPs should correspond to the final absolute accuracy the user needs. For example, for projects for which an accuracy of some meters is acceptable (e.g. fast assessment tasks), then the accuracy of the GCPs is NOT required to be of some centimeters. For projects for which the accuracy is very important (e.g. construction sites) then the GCPs should be measured with an accuracy of some centimeters in order to comply with the project requirements. In general, the accuracy of the GCPs should be slightly better than the expected accuracy of the final results.