Ground control points are used in conjunction with GPS data collected during the flight to increase the accuracy of the area being surveyed. By creating visible attributes within the area being flown and knowing an accurate GPS coordinate on these attributes, aerial photography is able to take this information and create a survey grade map. A great example of how much time and money can be saved by using photogrammetry and GCP’s as a replacement for standard surveying techniques. GCP's are a great way to increase the accuracy of any map that uses aerial captured images.
Post-marked control uses points of natural detail or physical markings on the image emulsion (Aerotriangulation). Such points are visible on the photograph and can be uniquely and unambiguously defined. The point selection is made on the basis of the type of point, that is 3D, horizontal or height.
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