Wednesday, November 09, 2016

GIS "Pro" Tips - Creating Streamlines from a DEM

One of the things I needed to do a few times for my GIS job was to create streamlines and watersheds for a specific area using DEM and LiDAR data. Below is my workflow for creating streamlines from a DEM using ArcHydro (a free and very useful toolbar from ESRI).

Creating streamlines:

Within the ArcHydro tool set do the following processes:

  • Identify all dams and lakes within the area which are going to affect water flow.
  • Create lines across the dams to simulate where the water will flow if the dam were not damming the water.
    • Take the Dam line file and Assign HydroID’s to the lines.
    • Convert the dams lines to ArcHydro Drainage Lines using “Create Drainage Line Structures”
      • I often get an error running this tool, even though the “DrainageLine” file that is created is complete. Check the DrainageLine file to make sure the number of streams is the same as the original file. If it is, you should be fine.
  • Take a DEM and run the following Terrain Preprocessing tools in order to condition the DEM:
    • DEM Manipulation --> DEM Reconditioning: 
      • Use the new “DrainageLine”/Dam lines as the AGREE Stream  
      • Leave values as default except set “Sharp drop/raise” = 100
      • This creates an AgreeDEM
    • Take the AgreeDEM and Fill the sinks: DEM Manipulation  Fill Sinks
      • Use default values for everything
    • Take the Fill grid and run the “Flow Direction”
    • Take the Flow Direction grid and run “Flow Accumulation”
  • Take the Flow Accumulation grid (“Fac” if you left the defaults) and change the Symbology. Change it to Classified with 2 classes. Then change the division of the two classes to around 5,000 or 10,000. The lower the number the more streams you are going to have. Play around with it until you get something that is useful and not overwhelming. Then make the 0 – 10,000 (or whatever) range no color, and the other range black (or whatever color you want to visualize it in). 
  • When you find a value that you like, run the “Stream Definition” tool. Set the Flow Accumulation Grid, the under Number of Cells, put the number you figured out in the previous step.
  • Take the resulting Stream grid, and run the “Raster to Polyline” tool in arcmap (not in ArcHydro unless I missed it somewhere). 
    • Input Raster is the Str grid
    • Field will be “Value”
    • Background value is “NODATA”
    • And check the simplify polylines option
  • Your output will be a Polyline shapefile of your stream network. You will want to take these streams and “fix” them to better match the DEM and aerial for the area you are working on. Bridges, dams, and other culverts mess up where the water should actually flow so these will need to be readjusted to what their flow patterns should actually be. Once all of the streams have been adjusted as they should be move on to calculating out the watersheds for each area.

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