Modern aerial cameras . Vertical photographs are normally acquired in overlapping pattern, so as to create a stereoscopic effect when adjacent pairs are viewed together. The overlapping views of the same ground area produce a parallax effect, which is also the basis for depth perception in human vision. This ability to perceive depth is quite useful for visual interpretation of air photos. Airphotos of the Earth's surface are not maps .
Photographs are single-point perspective views, and as such they contain geometric distortions.
Flying height varies with topography, however, so scale is not constant throughout a photograph. Vertical air photos can be used for ground measurements and map making. Photogrammetry is the art and science of accurate measurements based on photographs. Dimensions and locations of objects, height differences, and absolute elevations may be determined accurately. Modern topographic maps are usually produced on the basis of air photos.