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 DEM_data.qgz project topo_intro folder.

    1. The bottom group “raw data in 1×1 degree tiles” shows the difference between grid resolution data for the same 1×1 degree square.
      1. Note the difference in file sizes (these data come from the R drive, not your folder!)
      2. Note that I am showing a transparent pseudocolor DEM over a hillshade rendering of the same data, for each DEM.
    2. The next group up tab “comparing geographic and UTM cell sizes” shows GCS vs projected data and its effect on cell location and size.
      raster reprojection resampling
      I believe this shows both the geographic DEM and the UTM dem in a geographic map canvas

      These are 1 arc second data points (raster) in Lat/Long (geographic coordinate system or “GCS”) and same data projected using gdal Warp to a 30 m grid (raster) in UTM.

      1. We’ll talk about the geoprocessing  required to generate new data in a different projection using Warp (on following page).
      2. Zoom in to an edge and switch the project CRS from geographic to UTM and back to geographic, while turn the UTM layer on and off.
      3. Measure the cell size and notice the change of orientation