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Creating DEMs from SRTM data

The Shuttle Radar Topography Mission mapped global elevations at a 3 arc second (90m posting) resolution between 60N and 56S. This is relatively coarse for DEM usage but is sufficient in some cases. To create a DEM from this data:

  1. Obtain data from SRTM (http://srtm.usgs.gov/). This is most easily done via their Seamless server - click on "View and Download international data", then from the icons on the left select "Define download area by coordinates (icon looks like "X Y" above a ruler). A dialog will be displayed asking for the limits of the area you want to download. Enter these and click "Add Area". Another dialog will be displayed, probably saying that there are no available products in the area indicated (unless that area was in North America). Click on "Modify Data Request", and on the resulting page deselect any selected options and select "SRTM 3 arc sec - Shuttle Radar Topography Mission [Finished]" (bottom option at time of writing). Then click "Save changes and return to summary", and once the summary is displayed click on Download and save the resulting zip file somewhere. If it still says no products are available, check that your area is not over the sea or too far north or south, and that you have selected the correct product (only the SRTM 3 second data is available for sites outside North America).
  1. Unzip the downloaded zip file and descend into the directory structure to find the file w001001.adf - this holds the actual data, though you can't move it without the rest of the directory structure.
  1. Fire up Grass. Select a location in lat/long projection using WGS84 datum (create one if none avaialble), make a note of the selected location and mapset names.
  1. Import the SRTM data file:
    r.in.gdal input=w001001.adf output=srtm_data
    
  1. Quit Grass and then start it up again. Select a location using UTM projection for the target area or create one if none is available - make sure you've got the right UTM zone. Note you can also use whatever other projection you want, but this guide assumes you want UTM.
  1. Use a conversion utility (eg see the spreadsheet at http://www.uwgb.edu/dutchs/UsefulData/UTMFormulas.HTM#Spreadsheet) to determine the boundaries of the target flight in meters within the selected UTM zone. If the flight spans two or more UTM zones, you will have to calculate the offsets appropriate to work out the east and west boundaries of the area. eg. If western boundary is zone 32 and eastern boundary is zone 33, for eastern value use zone 32, add 750000 to value from spreadsheet

to find eastings relative to zone 32 bound.

  1. Set active region to calculated eastings/northings using:
    g.region n=<north val> s=<south val> e=<east val> w=<west val> res=50
    
  1. Use r.proj to convert from lat/long to UTM:
    r.proj input=<input_map_name> location=<lat/long_location> mapset=<lat/long_mapset> output=<output_map_name>
    
  1. If r.proj gives an error saying that the input map is outside the bounds of the current region, go back to step 6 and check your numbers

10 Output ASCII DEM from converted map as normal (see Creation of DEMs from NextMap data)