[GRASS-user] r.watershed in grass 6.1 and 5.4

[Dylan Beaudette]

On Thursday 17 August 2006 06:41, Maciej Sieczka wrote:
> Hans Hauska napisa?(a):
> > SECTION 5: Closing Maps.
> >
> > everything seems to work fine, but When it reaches Section 5 then it
> > just stands on one point and chews ....
> > I had it run for over 12 hours, but nothing happened in the final stage
> > and it never finished.
> > Has anyone been able to successfully run it?
> > Is there a special trick? Should I use a Linux version (that is possible)
> > I'd appreciate any hints or suggestions on the matter.
>
> This is a long-standing bug http://intevation.de/rt/webrt?serial_num=3112
>
> r.watershed completes fine on smaller input. On bigger it hangs (don't
> know what the exact threshold is though).
>
> Maciek
>

Helena recently commented on this:

> r.terraflow currently has only multiple flow directions option which
> should give better results for hillslope erosion. On floodplains it
> spreads water
> so it won't create a single cell wide stream. It will be worth trying
> to see how the results look like
> with r.usped. Note that it is faster only on massive grids - for
> regular sized grids
> (1000x1000) it may be the same as r.watershed.
> For stream extraction with depressions r.watershed gives more
> accurate results
> but it gets stuck for grids larger than approx 1500x1500 (depending
> how big your memory is).
> r.terraflow runs on 10,000x10,000 with no problems. Duke univ. team
> is working on new version,
> but I did not have a chance to try it yet and it has not been yet
> implemented in GRASS.
> Ultimately my plan was to replace all flow routing with r.terraflow
> but it looks like
> that won't be possible because some of the algorithms cannot be
> rewritten as I/O efficient,
> so we will keep r.watershed for now.
>
> I cc this to Dylan - regarding the r.watershed getting stuck on step
> 5, it may be just color table
> (it assigns a random color to each subwatershed and that search for
> colors gets stuck when
> there are too many-
> if somebody would be willing to look at it (modify the code and test it)
> I will be happy to help. Hamish identified the problem long time ago
> (I might have submitted a bug report)
> but I don't have enough time to explore it (I looked at the code, it
> may not be too complicated).
> When r.watershed gets stuck on step 2, that is when it cannot be done/
> fixed.
> I just looked at Hans' command and if he used threshold 75 with a DEM
> that is around 1000x1000,
> that is where his problem is - he should use much larger threshold
> (thousands at least).
>
> Helena

So it would seem that we need to find a better random color generator to 
r.watershed, or just disable the color table if num watersheds > some 
threshold. I'll poke around in the source later today.

Dylan

-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341