A Digital Terrain Model (DTM) is a 3 dimensional surface which can be used to generate contours and profiles. Terrain uses a Triangular Irregular Network or TIN model (this is sometimes referred to as a Delaunay Tesselation). An array of connected triangles (planar facets) is created by essentially "connecting the dots". The vertices of the triangles are points with known elevations. Each triangle in the DTM represents a planer facet. The outer extent of the triangles forms a polygon (triangle boundary). The elevation of any point inside this boundary is defined by the corresponding elevation of the triangle which encloses it.
The time required to calculate a DTM increases rapidly with the number of points. For large data sets (> 10000 points) this time can become significant.
