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Terrain Mapping and Analysis Chapter 12 Introduction Terrain mapping Land surface is 3-D Elevation data or z-data is treated as a cell value or a point data attribute rather than as a coordinate. Data for Terrain Mapping and Analysis Digital Elevation Model (DEM) Array of elevation points 7.5 minute USGS quads into 4 levels Level 1 RMS 7-15 meters Level 2 RMS of ½ contour interval Level 3 RMS of 1/3 contour interval not to exceed 7 meters What happened to Level 4? Relative and absolute errors Data for Terrain Mapping and Analysis Triangulated Irregular Network (TIN) Series of non-overlapping triangles Elevation values are stored at nodes Irregular distribution Sources: DEMs, surveyed elevation points, contour lines, and breaklines Breaklines are line features that represent changes of the land surface such as streams, shorelines, ridges, and roads Data for Terrain Mapping and Analysis Triangulated Irregular Network (TIN) Not every point in DEM is used Only points most important VIP (Very Important Points) algorithm Maximum z-tolerance algorithm Delaunay triangulation: all nodes are connected to their nearest neighbor to form triangles which are as equi-angular as possible. Borders are a problem Go beyond study area and clip to make best Terrain Mapping Contouring is most common method for terrain mapping Contour lines connect points of equal elevation (isolines) Contour intervals represent the vertical distance between contour lines. Arrangement of contour lines reflect topography Terrain Mapping Vertical profile shows changes in elevation along a line, such as a hiking trainl, road or stream. Terrain Mapping Hill shading is also known as a shaded relief or simply shading Attempts to simulate how the terrain looks with the interaction between sunlight and surface features. Helps viewers recognize the shape of land- form features on a map. Digital shaded-relief map of US Terrain Mapping Four factors control the visual effect of hill-shading Sun’s azimuth is direction of incoming light (0 to 360°) The sun’s altitude from horizon (0-90°) Surface slope (0-90°) Surface aspect (0 to 360°) Terrain Mapping Hypsometric tinting Applies different color symbols to represent elevation zones. Terrain Mapping Perspective View Perspectives are 3-D views of the terrain wherein the appearance is as viewed from an airplane. Viewing azimuth (0 to 360°) Viewing angle (0-90°) Viewing distance Z-scale is ratio between he vertical scale and the horizontal scale (exaggeration factor) 3-D draping of vector information Terrain Analysis Slope measures the rate of change of elevation at a surface location Aspect is the directional measure of the slope (degrees- 4 or 8 directions) Important for analyzing an visualizing landform characteristics Accuracy an issue If you want to try, use the worked examples in the text with Excel Terrain Analysis Surface curvature: convex or concave Viewshed analysis Viewshed refers to the areas of the land surface that are visible from an observation point or points. Watershed analysis Watershed is an area that drains water and other substances to a common outlet Terrain Analysis Watershed analysis Requires three data sets in raster format Filled elevation grid Flow direction grid Flow accumulation grid Grid vs TIN Different algorithms and type of output Can be converted from TIN to grid or grid to TIN TIN has flexibility of input sources: DEM, breaklines, contour lines, GPS data and survey data as well as user added elevation points. Elevation grid is fixed with a given cell size Grid vs TIN Computational efficiency with grid TIN gives sharper image How are they built and used?
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