Digital Topography 2

Digital Topography

What’s in a name—-DEMs, TINs, DLGs, DTMs, Point Clouds 

  • DTM – digital terrain model (general name for digital topography, involves representation/generalization)
  • DEM – digital elevation model (gridded representation of point topography)
  • DLG – digital line graph (of contours, but same format for rivers rivers, transport)
  • TIN – triangular irregular network (triangular “facets,” each of which has a constant slope and aspect). Also known as “nets,”
  • Point cloud – returns from a terrestrial or airborne LIDAR that give ground and vegetation elevation (we’ll cover these later)
    Some advantages of TINS include:

  • Fewer points are needed to represent the topography—less computer disk space needed.
  • Points can be concentrated in important areas where the topography is variable and a low density of points can be used in areas where slopes are constant.
  • Points of known elevation such as surveyed benchmarks can easily be incorporated
  • Areas of constant elevation such as lakes can easily be incorporated
  • Lines of slope inflection such as ridge lines and steep canyons streams can be incorporated as breaklines in TINS to force the TIN to reflect these breaks in topography.
    http://www.ian-ko.com/resources/triangulated_irregular_network.htm

Open the topo_data project demotopo folder.

    1. The first tab “lat long vs UTM” shows GCS vs projected data. We’ll talk about projection. These are 1 arc second data points (raster) in Lat/Long (geographic coordinate system or “GCS”) and same data projected to a 30 m grid (raster) in UTM.
    2. The second map tab “National Map downloads” shows the difference between grid resolution data for the same 1×1 degree square.
    3. The third map tab “Data Types” shows many different DTM versions.
      1. We will examine grid, TIN, and contour datasets.
      2. Compare these data to what’s available online
        1. select the “add data button” and from the “living atlas” select “Terrain.” (not “Terrain: something else”)
        2. increase the transparency of the layer
        3. “add data” and choose “Terrain: multidirectional hillshade” from the Living Atlas, make sure it is below the DEM
        4. Group these two layers like the existing DEM
    4. Fourth map tab is for later…. visibility analysis.