Just how many GCPs do you really need?
 I often get asked "Just how many GCPs do you really need? My answer is, it depends! The number of Ground Control Points (GCPs) will depend upon the number acres that are being flown, the shape of the site and what drone/camera/sensor is being used. The very minimum is 3 that most processing software needs to start rectifying the images. My minimum is 5, one in each corner and one in the middle for a 1 to 2 acre rectangular lot. For a flat 5 acre site at least 8 are needed as shown below. If there is a lot of terrain relief, then more GCPs are needed to accurately capture the grade breaks for computational purposes by the processing software.
 The algorithm that is used by the processing software to optimize the imagery to GCPs is based upon the least squares methodology. When the GCPs are balanced, you get a much better solution than if they are laid out in an odd location. It is about the strength and size of the triangle between GCPs. In the image below triangles A, D & F are stronger than B, C & D. When the DEM elevations were checked by changing GCP 802 and 803 to check points, there was a 0.20' error reported in the vertical.
 The above project was for a 2D only orthomosaic. GCPs 802 and 803 were set to test the 3D capability of the Mavic 2 Pro. Had this been a 3D project, then additional GCPs would have been placed mid point along the west and east parcel line to strengthen the layout. The targets can be anything that will be visible in the image. I used small paper plates painted pink for the above project. They worked out real well.
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