[GRASS-user] r.watershed: whats wrong?

[Markus Metz]

achim wrote:
> Hi all,
>
> I used r.watershed on a small test-region an all I produce
> for accumulation and flow-direction was -1 on the whole area
> except on the border-cells. Whats wrong?
>
> Curiously is that the first time I tried on my equal-area projection
> everything seemed normal. Before on ll it was wrong and after on that
> laea-projection it was like mentioned.
>
> I tried on grass63 and on grass64 (and different thresholds, new
> output-names...). Both the same. (On opensuse 11.1 64)
>   
May be you need to run
g.region rast=etopo1_fil
first?

You can not use the same raster as elevation and as depression, this 
will give the you results you observed, nonsense results.
The name etopo_fil suggests that you filled the elevation map first 
before using it as input for r.watershed. This can heavily distort flow 
accumulation and flow direction output, the input elevation map *must 
not* be filled for r.watershed. Input depression is only needed if there 
are real depressions in the area, depressions without outflow. The Dead 
Sea is such a real depression, ordinary lakes are not, they usually have 
an outflow.
More details are in the help page:
http://grass.osgeo.org/grass64/manuals/html64_user/r.watershed.html

Good luck,

Markus M

> r.watershed elevation=etopo1_fil at achim depression=etopo1_fil at achim
> threshold=100000 accumulation=r_watershed_etopo1_accu
> drainage=r_watershed_etopo1_dir basin=r_watershed_etopo1_basin
> stream=r_watershed_etopo1_stream half.basin=r_watershed_etopo1_halfbasin
> visual=r_watershed_etopo1_visual
> length.slope=r_watershed_etopo1_lenghtslope
> slope.steepness=r_watershed_etopo1_slopesteepness --overwrite
>
> Can anyone help?
>
> Best regards
>
> Achim
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