Ground Control Point

“We can conclude that in order to obtain high accuracy of the final products, a density of 1 GCP/200m^2 is necessary”. [Determining the Optimum Number of Ground Control Points for Obtaining High Precision Results Based on UAS Images]

“a uniform coverage of GCPs is preferential across the area of interest”. “errors from this source can enter the decimetre range in locations greater than ~100 m from the nearest GCP”. Ground-Control Networks for Image Based Surface Reconstruction: An Investigation of Optimum Survey Designs Using UAV Derived Imagery and Structure-from-Motion Photogrammetry well written, see discussion