Digital Terrain Model. This type of surface uses break lines in addition to spot heights to refine the model.  The breaks are identified from our orthophoto and delineated along terrain impacting features such as ridges, valleys, watercourses, road edges and structures.  When the spot heights are interpolated to create the terrain model or contour lines, the terrain disruptions at the break lines are taken into account to produce a more true to life representation of the surface.