[GRASS-user] Watershed delineation

[Jaime Carrera]

Hi Daniel,

Well, r.watershed does have a logic behind the way in the numbering scheme. What you can do is wait for r.watershed to end and then using its watersheds delimit the r.terraflow watersheds.
By the way, if you want to delineate the watersheds, it's better to run r.watershed on smaller areas (and using a larger threshold) a couple of times; you'd only have to wait at most a couple of hours. Then patch the maps and create your watersheds (using also r.terraflow's output); I did this for a 9600 km2 basin.

Jaime

Daniel Victoria <daniel.victoria at gmail.com> escribió: I tried searching through the mailing list for something about
r.terraflow and the logic in the numbering scheme but could not find
anything. Could some kind soul brieflly explain the logic behind it?

 I did find a message between Dylan and Hamish last year about joining
the basins output from r.terraflow. Basically trya mixture of r.drain,
v.extract or r.flow..

http://article.gmane.org/gmane.comp.gis.grass.user/15113/match=terraflow+logical

Hamish, have you tried using r.drain to connect the smaller basins?

I tough about using r.terraflow with the D8 option and the use the
flow direction output with r.water.outlet. Is that possible?
Unfortunatelly I can't test it here because r.watershed is still
running (+2 days) and I don't want to stop the process now that it
reached 50%...

Cheers
Daniel

On Dec 28, 2007 7:12 AM, Hamish  wrote:
> Daniel Victoria wrote:
> > > I trying to delineate watersheds for several points in a very big
> > > DEM
> > >
> > >  | Data Type: CELL |
> > >  | Rows: 4803 |
> > >  | Columns: 5172 |
> > >  | Total Cells: 24841116
> > >
> > > I've successfully ran r.terraflow on the DEM but now I'm stuck.
> > > How do I get the watershed area for my points of interest? I've
> > > tried using the direction map produced with r.water.outlet but it
> > > did not work (I used MFD flow calculation).
> > >
> > > Also, from what I understood, the sink watershed map is suposed to
> > > be the watersheds delineated by the terraflow module, is that
> > > correct?
>
> Near-to, but not exactly.
>
> > > Anyway, I think I'll let r.watershed run for the night
>
> For a region that big r.watershed will take a long time to finish, but
> it can be worth the wait. IIRC you get more accurate results from
> r.watershed than from r.terraflow. (see historical post by ?
> Helena/Laura/Charles?)
>
> The hardest part with r.watershed is getting the threshold setting near
> to the scale you wish to work with.
>
> For watershed delimination see (new) examples here:
>  http://grass.ibiblio.org/grass63/manuals/html63_user/r.watershed.html
>  http://grass.gdf-hannover.de/wiki/Psmap_flooding_example
>
>
> Jaime Carrera wrote:
> > > that's right; you end up with a lot of sub-basins, which you can
> > > aggregate according to your need. They are numbered on a logical
> > > sequence, so it's really easy to aggregate them with r.mapcalc;
> > > just check where a key outlet is and go upstream from there.
>
> Daniel:
> > Well, I think that something has gone wrong then because the basin
> > numbering does not seam to be in a logical sequence. Or I have not
> > understood what is the logic in the numbering sequence.
>
> AFAIR Jamie is correct, there is some logical sequence there. I don't
> see much about it in the help page but if you search the mailing list
> archive or source code you can find it.
>
> > Just for the record, here is my g.region -p output:
> ...
> > nsres:      899.99917697
> > ewres:      899.9166173
> > rows:       4803
> > cols:       5172
> ...
> > The resolution is strange because the DEM was projected from a 30
> > arcsec latlon map. I tried to set it to 900 meters but the output was
> > like this...
>
> I think there is a way to have r.proj use the target location's region
> setting (region cropping flag?). The ugly way to clean the resolution
> is to force a 900m res with 'g.region res=900 -a'
>
>
>
> good luck,
> Hamish
>
>
>
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