Creating DEM from PASDA las files

The following is a description of how the maps discussed in the previous post were constructed. This information is provided in the spirit of open access and replicability. The following is a step-by-step guide to building digital elevation models (and their derivatives) from PASDA LiDAR data.

  • Download las tiles from PASDA.
    • Go to PASDA Imagery Navigator: http://maps.psiee.psu.edu/ImageryNavigator/
    • Zoom in on the area of interest.
    • Under the Display Tile Index drop down menu, select “Lidar Hillshade”
      • This will show you the tile index and the relevant file names
    • Place your cursor over a spot of interest and right click.
      • This will bring up a list of available data.
      • Click on the “LiDAR,, Topo and DEM” tab
      • At the right, you will see a listing of “LAS” files for download.
    • Select and download all the appropriate files.
  • Convert projection and reserve only category “2” points (2= ground return).
      • Note that Pennsylvania Data MUST be converted from NAD83 PA S (feet) to NAD83 PA S (meters)
      • Open las2las.exe
        • In the upper left, find and select all of the files from the above.
          • Note that you can use the wildcard (.las, not .laz as is the default)
        • Keep only ground points
          • Expand the “filter” menu
          • Select “keep_classification” under “by classification or return”
          • Under “number or value”, enter 2
        • Reproject from feet to meters.
          • Under “target projection” select
            • State plane 83
            • PA_S
            • Be sure “units” are in meters.

Your GUI should look something like this:

1las2las

      • Choose an output location in the upper right.
      • Click “Run” (in the lower left; you may have to minimize (click the “-“))
      • In the command line, you should see something like:
        • las2las -lof file_list.7808.txt -target_sp83 PA_S –olaz
      • You should now have reprojected las files that include only the ground return.
    • Convert las files into smaller “tiles”
      • Open “lastile.exe”
      • Add the reprojected las files (actually now they should be laz files) in the upper left.
      • Choose a tile size of 1000 (for the above this means 1000 meters)
        • Choose a buffer of 25 (you need a buffer and just need to experiment with what works best for you.)

Your GUI should look like this:

2lastile

    • Hit “Run”
    • The command line should look something like this:
      • lastile -lof file_list.1576.txt -o “tile.laz” -tile_size 1000 -buffer 25 -odir “C:\Users\Benjamin\Desktop\Working_LiDAR\Repoj_tile_las” –olaz
  • Convert tiles into DEM
    • Open “BLAST2DEM.exe”
    • Add the tiles constructed in previous section
    • Choose your output location
    • Choose “tif” for file format

Your GUI should look like this:

3blast2dem

    • Click “RUN”
    • Your command line should look like this:
      • blast2dem -lof file_list.6620.txt -elevation -odir “C:\Users\Benjamin\Desktop\Working_LiDAR\DEM_tiles” –otif
    • Your DEM’s are now created.

From here, you will want to stitch the DEM’s back together, but you need a GIS program for that. You can use the open source QGIS.

  • Open QGIS
  • Click on Raster- Miscellaneous- Merge.
  • Select the “choose input directory instead of files” box
  • Select the destination location and file name.
  • Click “OK”-
    • I frequently get an error here, but the results appear complete.

At this point, all of your data should be in a single Geotiff file (be sure to save it) as a digital elevation model.

In order to complete the analysis in the previous post, I converted the DEM into a slope model, which shows high slope in lighter gray and low slope in darker gray.

  • To do this, all you need to do is, in QGIS, use Raster- Terrain analysis- Slope. The input is your DEM and the output is the new slope model.
  • Within QGIS, you should now be able to see maps similar to those shown in the previous post.

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