These DTMs are stored in ASCII text format and contain five columns of data: Distinctions between 'hard' and 'soft' breaklines were established and standardized to facilitate the generation of contours. Spot elevations at summits and in depressions and breaklines along significant linear features were also collected. Mass points were collected along parallel scan lines 75 meters apart at variable density as a function of the topography and other ground features. The DTM data points were collected on analytical stereoplotters at a sufficient density to support generation of 3-meter contours conforming to the National Map Accuracy Standards (+ or - 1.5 meters). The DTMs available from MassGIS were produced for the processing of the 1990's Aerial Images and their related data layers ( Elevation Contours , Elevation Points , and Topographic Breaklines). They are used as input for the generation of surface models and contours or the orthorectification process of aerial photography. Digital terrain models are representations of land surface point elevations.
