The types and number of control or reference points that a given type of base map will provide. There are several types of materials that can be used as base maps, but three or commonly used. Vertical areal photographs. Vertical photographs have an advantage over oblique ones in that features on the landscape cannot be obscured by other features. They also benefit from not being as distorted.
Distortion does, however, increase toward the margins of the photos.
They are particularly helpful if they are stereoscopic. Vertical aerial photographs are normally taken in parallel overlapping strips. By virtue of the overlap, these photographs provide a three-dimensional perspective when viewed with a tool called a stereoscope. They are available for many parts of the United States from several government agencies and private commercial concerns. The single best source of information about obtaining aerial photographs is the Earth Science Information Center, or ESIC .