Orthorectification
From OpenAerialMap
Orthorectification is a processing step in which aerial photos are adjusted to correct distortions due to topographic relief, lens distortion, and camera tilt
These geometrical corrections should mean that the scale of the photograph is uniform, such that the photo is equivalent to a map. Unlike an uncorrected aerial photograph, an orthophotograph can be used to measure true distances, angles and areas. As such it can used to derive a map (very useful for projects like OpenStreetMap)
