Ground control points - What for? How many?

Scene Reconstruction leads to a 3D coordinate space which is made up of arbitrary model-coordinates. These have to be transformed into map coordinate space. To do so, we need reference points, for which the coordinates of your preferred coordinate reference system must also be known.

Your part is, to supply us with the map coordinates for the reference points. You can get the X-, Y- and Z-coordinates either from in-flight GPS-recordings of your camera positions or from field-measurements of airphoto-visible points.

With proper ground control points you can expect a much higher positional accuracy.

For us it would be fine to have at least 4 - 6 uniformly spreaded ground control points per block of 250 - 400 images. The achievable overall accuracy depends on the accuracy of your GCPs, the image quality and the overall terrain charakteristics.