[GRASS-dev] Re: [GRASS GIS] #518: negative flow accumulation with r.watershed SFD or MFD

Dylan Beaudette debeaudette at ucdavis.edu
Fri Mar 6 14:35:12 EST 2009

On Fri, Mar 6, 2009 at 12:18 AM, GRASS GIS <trac at osgeo.org> wrote:
> #518: negative flow accumulation with r.watershed SFD or MFD
> --------------------------+-------------------------------------------------
>  Reporter:  dylan        |       Owner:  grass-dev at lists.osgeo.org
>      Type:  enhancement  |      Status:  new
>  Priority:  major        |   Milestone:
>  Component:  default      |     Version:  svn-develbranch6
> Resolution:               |    Keywords:  r.watershed
>  Platform:  Linux        |         Cpu:  x86-32
> --------------------------+-------------------------------------------------
> Comment (by mmetz):
>  IMHO, there is still some cleaning up to do for r.watershed. I left some
>  things in it for backwards compatibility. One such thing is the "visual"
>  output which I regard as obsolete because "accumulation" output now comes
>  with a (better I hope) colortable by default.
>  The "visual" output could be removed and another output option be added,
>  e.g. called "absacc" that gives absolute accumulation values. That would
>  however break backwards compatibility, a new flag would not.
>  There is a good reason *not* to add this option/flag, nicely illustrated
>  by Dylan creating this ticket. The purpose of negative accumulation values
>  is to make people wonder what on earth is going here, then figure out that
>  not the whole catchment area under study was included and expand the
>  computational region accordingly to get proper results: only positive
>  accumulation values for the catchment under study.

I like your approach Markus, but I think that we should test this out
a bit more-- as I was seeing negative values in areas that honestly
should be part of the region in question. I will check on my data, and
post back with some examples. It could be that my elevation model has
some subtle problems.

I will address this in another ticket, but for the record I have seen
some odd contour-like artifacts in the flow accumulation raster-- but
only after resetting the color map using the '-a' flag to r.colors.

PS: the speed improvements in r.watershed are awsome. Thanks for all
the hard work!


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