A post on the Ordnance Survey’s blog explains how they acquire and process aerial photography. “We have a large contract in place with external suppliers to supplement our own flying and photogrammetric production. This gives us the capacity to have to 6 planes flying on our behalf at any one time, allowing us to make best use of good weather conditions and process 60 000 to 70 000 sq km (more than a quarter) of Great Britain each year.”