DEM (Digital Elevation Model)

 

http://ned.usgs.gov/Ned/historic.asp 

A Digital Elevation Model (DEM) is a three-dimensional representation of a terrain's surface using elevation data.  A DEM file is a simple, regularly spaced grid of elevation points used to create a DEM graphic. The above image is a DEM of Mount Saint Helens after it erupted on May 18, 1980. When compared to the image from before the eruption, the extent of the damage can be observed. The whole top half of the original mountain is gone in the above image.