[GRASS-user] r.basin does not complete properly on one stream

   For a reason I've not been able to determine in a long day today r.basin
fails to properly complete when passed easting/northing coordinates of a
cell generated by r.stream.extract. The last three ratios are shown as NAN
and the area is shown as only 4km^2 when it's actually much larger. With
other sub-basins this result was corrected by moving the outlet point
further upstream from the confluence with a larger stream.

   Following the r.basin wiki that madi wrote, I re-extracted the streams
from the DEM. Ran r.watershed to generate an accumulation map, then
r.stream.extract. Both modules take the same DEM map (named dem_proj). But,
when I place the outlet point several cells above the confluence r.basin
does not properly complete. I tried moving the outlet away from the
confluence, but still no joy.

   The attached PDF shows the outlet point sitting in the middle of a cell on
the stream_network raster generated by r.stream.extract.

   I'm at a loss to understand why this one sub-basin is fighting me, and I
would like outside opinions on what might be the cause and suggestions of
what to try to isolate the cause of this problem. If more information is
desired, I'll try to provide it.

Rich

(attachments)

stream-map.pdf (8.55 KB)

Hi Rich,

unfortunately I don’t have enough information in order to help you, would it be possible to provide a sample data?

thanks and regards,
madi

On Fri, Oct 26, 2012 at 1:40 AM, Rich Shepard <rshepard@appl-ecosys.com> wrote:

For a reason I’ve not been able to determine in a long day today r.basin
fails to properly complete when passed easting/northing coordinates of a
cell generated by r.stream.extract. The last three ratios are shown as NAN
and the area is shown as only 4km^2 when it’s actually much larger. With
other sub-basins this result was corrected by moving the outlet point
further upstream from the confluence with a larger stream.

Following the r.basin wiki that madi wrote, I re-extracted the streams
from the DEM. Ran r.watershed to generate an accumulation map, then
r.stream.extract. Both modules take the same DEM map (named dem_proj). But,
when I place the outlet point several cells above the confluence r.basin
does not properly complete. I tried moving the outlet away from the
confluence, but still no joy.

The attached PDF shows the outlet point sitting in the middle of a cell on
the stream_network raster generated by r.stream.extract.

I’m at a loss to understand why this one sub-basin is fighting me, and I
would like outside opinions on what might be the cause and suggestions of
what to try to isolate the cause of this problem. If more information is
desired, I’ll try to provide it.

Rich


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Margherita DI LEO

Postdoctoral Researcher
European Commission - DG JRC
Forest Resources and Climate
I-21020 Ispra (VA) - Italy - TP 261

Tel. +39 0332 78 3600
margherita.di-leo@jrc.ec.europa.eu

On Thu, 25 Oct 2012, Rich Shepard wrote:

For a reason I've not been able to determine in a long day today r.basin
fails to properly complete when passed easting/northing coordinates of a
cell generated by r.stream.extract. The last three ratios are shown as NAN
and the area is shown as only 4km^2 when it's actually much larger. With
other sub-basins this result was corrected by moving the outlet point
further upstream from the confluence with a larger stream.

   For 5 of 7 sub-basins applying a threshold of 7500 to r.basin worked fine.
For the other two, adjacent with nearby outlets, the threshold needs to be
reduced to 2500 to have r.basin run to completion and produce two separate
basins without overlap.

   Problem resolved.

Rich