Digital Elevation Model (DEM)

(glossary entry)

Description

A digital representation of height information for a surface.

Lunar DEM's can be presented as data files in which a succession of numbers record the elevation at discrete steps in latitude and longitude, or as images in which the intensities and/or colors encode the heights at the corresponding pixel locations. Grayscale maps seem less attractive than the colored ones, but they are much more precise for reading of the elevations especially if a linear scale is used with each of the 256 possible intensity steps (in a standard grayscale image) corresponding to a specified height.
DEM information is typically derived by sounding the surface from a spacecraft at known altitude (usually using a laser-based device known as a LIDAR), from stereo mapping (greater discrepancies in height being reflected in greater displacements in position when viewed from two angles), or by inference from the observed brightness of the surface as viewed from a number of angles and interpreted as an indication of its slope.