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