![](http://www.solarviews.com/raw/mars/marscyl2.jpg)
DEM is a digital representation of ground surface topography and/or terrain. They can be represented as a raster or as a triangular irregular network. They are used often in GIS, and are the most common basis for digitally-produced relief maps. This digital elevation map of Mars was produced by the USGS from Viking image data.
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