On Mar 6, 2009, at 3:09 AM, <grass-dev-requ...@lists.osgeo.org> wrote:
Date: Fri, 06 Mar 2009 08:18:38 -0000
From: "GRASS GIS" <t...@osgeo.org>
Subject: [GRASS-dev] Re: [GRASS GIS] #518: negative flow accumulation
with r.watershed SFD or MFD
To: undisclosed-recipients:;
Message-ID: <048.30fedfc30b8b5f67edb5ee82855f1...@osgeo.org>
Content-Type: text/plain; charset="utf-8"
#518: negative flow accumulation with r.watershed SFD or MFD
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+-------------------------------------------------
Reporter: dylan | Owner: grass-dev@lists.osgeo.org
Type: enhancement | Status: new
Priority: major | Milestone:
Component: default | Version: svn-develbranch6
Resolution: | Keywords: r.watershed
Platform: Linux | Cpu: x86-32
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+-------------------------------------------------
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.
IMHO, this is a very poor way to achieve this end--i.e., silently
performing all hydrology operations and producing completely bogus
values to get you to scratch your head and wonder what is going on
when you notice it. I suspect this is a legacy of the age of this
module. To make things worse, GRASS's other hydrology modules do not
work this way.
A much more direct way is to give a warning for each problematic basin
in the output:
WARNING: part of basin XX extends beyond region extent; accumulation
values may be too low.
It should be possible to turn off this "feature" of negative
accumulation. This is especially important for scripting. For example,
we use accumulation values as part of a complex, iterative erosion/
deposition model. We are currently running this on complete watersheds
created with r.watershed (although an earlier version). Our goal is
not to create accumulation maps and in fact never see the accumulation
maps but use the values for additional modeling. Watershed maps get
deleted along the way unless a flag is set to keep them because of the
very large numbers of maps created to model decades or centuries of
surface process dynamics. The new version seems to be calculating the
watershed in a slightly different way, and now a tiny bit must extend
off the region because we are getting negative values in some
watersheds that were not problematic before. We will check this of
course. But we never knew that we were getting negative values until
this issue came up in another context. So our model values are
completely bogus and we have to run this over again. The slight
difference in accumulation would have only a very tiny effect on our
results, but entire negative accumulation values make a big
difference. So we'll have to build in taking the absolute value of
accumulation (completely negating the goal of the negative values,
BTW), but it would still be nice to have a text informational warning
about which watersheds might be a problem for users of the model.
Michael
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