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<br />Guidelines and Specifications/or Flood Hazard Mapping Partners [Apri/2003} <br /> <br />. Mass points; <br />. Breaklines; <br />. TINs; and <br />. DEMs <br /> <br />These models are discussed in Subsections A.4.2.1 through A.4.2.5. <br /> <br />A.4.2.1 <br /> <br />Digital Contours <br /> <br />[February 2002] <br /> <br />Digital contours, or hardcopy plots of such contours, are normally used in manual hydrologic and <br />hydraulic analyses. Contours normally depict breakline features of interest to hydraulic engineers. <br /> <br />A.4.2.2 <br /> <br />Mass Points <br /> <br />[February 2002] <br /> <br />LIDAR produces irregularly spaced mass points. "First-return" LIDAR data provide elevation <br />mass points on reflective surfaces (e.g., treetops, rooftops, towers). "Last-return" LIDAR data <br />provide elevation mass points of the bare-earth terrain, but only after successful completion of <br />automated and manual post processing for vegetation removal and cleaning (removal) of manmade <br />features and artifacts. Mass points can also be produced when using photogrammetric methods. <br /> <br />A.4.2.3 <br /> <br />Breaklines <br /> <br />[February 2002] <br /> <br />Breaklines are linear features that describe a change in the slope, smoothness, or continuity of a <br />surface. "Soft breaklines" ensure that known z-values along a linear feature are maintained, and <br />they ensure that linear features and polygon edges are maintained in a TIN surface model, by <br />enforcing the breakline as TIN edges; but, they do not define interruptions in surface smoothness. <br />"Hard breaklines," which define interruptions in surface smoothness, are used to define streams, <br />shorelines, dams, ridges, building footprints, seawalls, and other locations with abrupt surface <br />changes. Automated techniques for generating digital elevation data, from LIDAR data for <br />example, normally do not generate breaklines well. <br /> <br />Breaklines are best produced either by stereo photogrammetric procedures, where three- <br />dimensional breaklines are produced (coordinate line strings having x/y/z coordinates), or digital <br />orthophotography, where shorelines, stream centerlines, and other breakline features are digitized <br />as two-dimensional breaklines (i.e., coordinate line strings having x/y coordinates but no z-values). <br />Regardless of the technology used, the Mapping Partner shall normally produce breaklines for <br />stream centerlines, drainage ditches, tops and bottoms of streambanks, ridge lines, road crowns, <br />levees, bulkheads, seawalls, roadlhighway embankments, and selected manmade features that <br />constrict or control the flow of water (e.g., curb lines). The Mapping Partner also shall specifY the <br />sources and accuracy ofbreakline data. <br /> <br />A.4.2.4 Triangulated Irregular Networks [February 2002] <br /> <br />A TIN is a set of adjacent, non-overlapping triangles, computed from irregularly spaced points with <br />x/y coordinates and z-values. The TIN data structure is based on irregularly spaced point, line, and <br /> <br />A-II <br /> <br />Section A.4 <br />