A digital elevation model DEM is a digital representation of ground surface topography or terrain. It is also widely known as a digital terrain model DTM. A DEM can be represented as a raster a grid of squares or as a vector based triangular irregular network TIN. DEMs are commonly built using remote sensing techniques but they may also be built from land surveying. DEMs are used often in geographic information systems and are the most common basis for digitallyproduced relief maps. The terrain