I'm new to GIS - more so in GRASS GIS. I installed GRASS GIS 6.4.3svn in my Windows 7 machine. No problem in installation.

  • Downloaded SRTM-Geotiff file
  • Location, Mapset all o.k
  • Used r.in.gdal to import SRTM file
  • Filled the sinks using r.terraflow
  • Masked the filled raster
  • Zoomed the required region & set the computational region
  • Ran r.watershed using threshold=10000
  • Got drainge, basin, stream & visual rasters
  • Visual rendering of rasters (drainge, basin, stream & visual) - o.k
  • Different Watersheds (basins) displayed for entire area
  • Zoomed stream raster to find lat/lon of mouth of river (cross checked with Google earth)
  • Converted the lat/lon as decimal
  • Ran r.water.outlet using stream as input & gave the lat/lon as easting & northing
  • Sadly the output display is blank
  • Rerun r.water.outlet - only a small dot in the lat/lon point. No delineation of watershed for the given easting & northing
  • All the above procedures as per manuals
  • Self learning of GRASS GIS - no formal training

Since I'm new here, I'm not permitted to upload the result images - Sorry...

What is the mistake I'm doing? How to get the watershed (basin) for the given lat/lon? Kindly help. Thanks in advance...

  • 4
    Well friends, I'm answering my own question. The procedure I followed is almost correct. The mistakes which I have done - hope it helps new comers like me... - Used stream raster instead of drainage - in r.water.outlet - please be careful in the input - Entered lat/lon with 2 digits after decimal - for accuracy atleast 4 digits after decimal is required. - Keep the stream vector displayed & adjust the lat/lon till you get watershed - by gradual increase / decrease of lat/lon. Thanks to all ..... – Vishy Sep 22 '12 at 17:47
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    Important: no sink filling needed for r.watershed! It does least cost routing. Hence no need to use r.terraflow here, you can simplify your procedure. – markusN Sep 22 '12 at 19:46
  • Also, the input to r.water.outlet should be the flow direction raster (From r.watershed, the "drain=" output parameter). – Micha Sep 23 '12 at 6:29
  • Thanks to markusN & Micha for important & useful reply. Is it necessary to provide such a accurate easting & northing (4 digits after decimal)? Somewhere I saw, one GIS user provided just two digits after decimal & the watershed plotted nicely. Kindly enlighten me.. – Vishy Sep 23 '12 at 15:08
  • It doesn't matter how many digits after the decimal point. What matters is that the outlet point falls exactly on a stream. If the outlet points happens to fall exactly on a degree then you will not need any places after the decimal. But in most cases you probably should use at least 4 places, as you wrote. – Micha Sep 23 '12 at 18:08

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