[GRASS-user] Overlapping vector areas

[Micha Silver]

On 07/29/2010 06:38 PM, David Townshend wrote:
> Hi
>
> I am trying the generate watersheds for several outlets by looping
> over r.water.outlet in a python script (attached). The basin procedure
> is that for each outlet point (read using v.info -t), my script runs
> r.water.outlet, r.to.vect and v.patch, to combine the watersheds into
> a simgle vector map. I am connecting the watersheds to the outlet
> points by setting the category value in the output from r.to.vect,
> i.e. before patching. The trouble is that some of the watersheds
> overlap, e.g. where an outlet lies inside another watershed. v.patch
> doesn't seem to handle this too well, and the result is that I have a
> vector map with a few areas, a few boundaries, a few centroids and a
> lot of errors.
>
> Is there a better way of approaching this? And how do I allow overlapping areas?
>
>    
No, since GRASS enforces topology, there's no such thing as overlapping 
polygons.  If you run r.water.outlet twice in the same stream network, 
you will get one basin that's a sub-catchment of a larger basin. If you 
try to patch these two vector maps, assuming the patch succeeds, and 
there are no topology errors, you'll get the smaller basin as one 
polygon, and the larger basin with the smaller one "cut out" as the 
second polygon.

I've encountered a similar problem to yours: many small overlaps or gaps 
along the boundary between two adjacent basins created by two runs of 
r.water.outlet.
I don't know of any easy solution, other than using the options included 
with "v.clean" to remove small areas and snap boundaries that are within 
a given threshold. THen after v.clean, there will often still be a few 
errors that need to be ironed out manually with v.digit.

HTH,
-- 
Micha
> Regards
> David
>
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-- 
Micha Silver
Arava Development Co. +972-52-3665918
http://surfaces.co.il

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