[GRASS-user] r.watershed: output basin map

[Rich Shepard]

On Thu, 19 Apr 2012, Margherita Di Leo wrote:

> If I understand, the blue lines are the "natural" rivers, which may differ
> from the calculated stream network. Maybe could be useful to display the
> calculated stream network in order to understand the logic that the
> algorithm may have followed creating those basins. Further, I would also
> give a try using r.stream.basins.

madi, et al.:

   I've learned something very useful in pursuing this thread; perhaps it
will be of value to others, too.

   Even though I ran 'r.mask -r' to remove the mask on the DEM, it persisted
and caused r.stream.extract to choke and die because the accumulation map's
region did not match the DEM's region. The reason turned out to be the
unintentional mask.

   So, I started over with the overall DEM at 0.305m resolution. With 40
gigacells it was too large to process on this workstation with "only" 4G
RAM. So was a change to 5m resolution. But, 10m resolution works. (Later
I'll re-run the analyses on my laptop with 8G RAM.)

   My analytical areas are the four 12-digit hydrologic unit sub-watersheds
outlined in black on the attached map. So I changed the region:

g.region vect=basins align=dem_proj

then ran r.watershed with the same 150,000 cell threshold as before.
However, the output is quite different now as you can see on the map. The
resulting basins are too numerous to make broad generalizations about
differences; I think that I need to greatly increase the threshold value.

   I've no idea how the DEM retained the mask outline after I removed the
MASK file, but it certainly did and that flawed the model results.

Thanks all!

Rich
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ebeco.pdf
Type: application/pdf
Size: 84347 bytes
Desc: 
Url : http://lists.osgeo.org/pipermail/grass-user/attachments/20120419/8241f338/ebeco-0001.pdf