Airborne GPS and Inertial technology has improved to the extent that for many aerial survey projects, ground control is no longer necessary. However, for any survey that has to tie in with a national system or to prior survey work OR where the ultimate precision is required, ground control is still necessary. In the case of UAV systems that generally cannot carry precision GPS hardware, it is always necessary. And lots of it.
Control can be carried out before photography by marking the ground with painted marks. These marks have to be of a suitable size matched to the photographic specification.
Alternatively control can be surveyed after photography by fixing points of detail visible in the photography. This is a more error prone technique and usually involves twice as many fixes to guard against mistakes.
Ground control is invariably carried out via GPS fixes which are derived from a geodetic framework of one sort or another.
Note that some of the pictures in the slide show are of some quite old projects but we hope they are entertaining.