Digital Elevation Models are data files that contain the elevation of the terrain over a specified area, usually at a fixed grid interval over the “Bare Earth”. The intervals between each of the grid points always refer to some geographical coordinate system. A DEM can also be represented as a raster (a grid of squares, also known as a heightmap when representing elevation) and therefore is very functional for orthophoto production and other mapping applications. A DEM is produced from a Stereo Pair.