A digital elevation model or D.E.M. is a three dimensional representation of a terrain's surface which can include non-terrestrial surfaces like the moon. D.E.M. data is mostly received from satellites. Multiple satellites coordinate and perform stereo and tri-stereo operations for accurate elevation readings. Stereo satellite operations use two satellites to create D.E.M.s when there is no large change in elevation heights whereas tri-stereo operations use three satellites that can accurately show data between areas where there is a large change in elevation height. Private companies as well as governmental satellites can provide this type of imaging which vary in image quality and resolution. The image shown above is a digital elevation model of a small part of Indiana where you can distinguish the ridge lines that could form a river from the rest of the landscape which is at a higher elevation.
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