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I'm new to GRASS. We want to delineate a watershed using the LIDAR data we have our hands on. I tried to follow several procedures to do so, but despite all my efforts, I can only get it to select a single cell out of the DEM.

I'm using GRASS 7.0.5

What I've done:

  • Created the DEM using r.patch and the necessary tiles
  • Defined my work region (g.region)
  • r.patch the DEM to fix the bridges
  • Used r.watershed to create the Accumulation and Drainage direction layers (tried with 0, 5k and 50k thresholds)
  • Used the Drainage layer to r.water.outlet

While I eventually get a result, it is only a single pixel, although I made sure to be very (VERY) precise (5 digits precise...) on the stream coordinates. Picture of the output.

enter image description here

I believe part of the problem is that my DEM is huge... but I find it hard to believe that I'm the only one using this kind of data.

Any thoughts?

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    What is the diagonal line in the image above? Note that the outlet coordinates for r.water.outlet must be on the stream network created by r.watershed.
    – Micha
    Jan 12, 2017 at 21:54
  • The line (diagonal) is my stream network, but my vector layer. I indeed made the mistake of not using the stream generated by r.watershed. I'll give it a try and come back with the results, thanks!
    – Horizen
    Jan 12, 2017 at 22:21

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As mentionned by Micha, my problem was that I was using my original stream vector layer instead of the one generated by r.watershed.

So to be clear: - With r.watershed, create both Drainage direction and Stream (at least) - With r.water.outlet, use the Stream raster layer generated this way to pinpoint precisely your outlet and the Drainage direction layer as raster input for computation

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