GCP placement is critical!
 What is the maximum distance between GCPs?
 The placement of GCPs is critical to being able to process a good aerial triangulation solution. The camera/sensor and type of lens will determine just how many and what type of spacing is needed. For 2D orthomosaics the number of GCPs can be less and the location is less critical. For 3D survey mapping it is more critical that the GCPs be set in strategic areas to get the best possible solution. It really comes down to the strength of the basic triangle when using trigonometry that we were taught in high school. A balanced triangle is stronger than a triangle with two long sides and a very short side. The same is true when placing GCPs. If they are balanced then the algorithum that is used by the photogrammetry software will produce a much better result. The distance between the GCPs also needs to be within acceptable tolerances to produce a good solid solution. I have seen way too often where GCPs are just thrown out without any thought to the strength of the triangle or the number of GCPs needed for a given sized parcel. To illustrate the point the image below shows a 6 acre parking lot with 6 GCPs placed at strategic points to produce strong triangles
 The GCP outside polygon is critical as well. A GCP needs to be set in each of the corners of the flight limits so that the aerial triangulation solution is constrained at these points. A GCP in the middle is also needed to constrain the model to provide a balanced triangle between GCPs.
 The following aerial triangulation solution was ran holding the outside GCPs with the one in the middle (Point 105) used as a check point.
 As you can see there is a vertical error of over 0.6' when the middle point was not used to constrain the model. This will produce a 3D model that is out of vertical tolerance.
 Adding Point 105 to the constrained model, you will produce a much better result.
 Conclusion The above example illustrates the fact that the location and number of GCPs is critical in getting a good solid aerial triangulation solution for 3D survey mapping.
 The above formula is the solution I have come up with to determine the number of GCPs that are needed given the flight limits in acres. It has been working pretty well so far. I am working on a comprehisive study to determine the maximum distance between GCPs before the vertical component produced by the aerial triangulation is out of tolerance. The results of that test will be made available some time later this year.
 www.cc4w.net "I fly Drones for a living mapping the earth one (1) acre at a time and write DIY surveying math/mapping books/videos for fun!"
 Check out the New video series on "Survey Mapping Made Simple".