#518: negative flow accumulation with r.watershed SFD or MFD
-------------------------+--------------------------------------------------
Reporter: dylan | Owner: grass-dev@lists.osgeo.org
Type: defect | Status: new
Priority: major | Milestone:
Component: default | Version: svn-develbranch6
Keywords: r.watershed | Platform: Linux
Cpu: x86-32 |
-------------------------+--------------------------------------------------
Hi,
I just noticed that there are negative flow accumulation numbers in the
output from the recently updated r.watershed. I am working with today's
SVN (develbranch_6). Here is an example from the spearfish dataset:
#518: negative flow accumulation with r.watershed SFD or MFD
----------------------+-----------------------------------------------------
Reporter: dylan | Owner: grass-dev@lists.osgeo.org
Type: defect | Status: new
Priority: major | Milestone:
Component: default | Version: svn-develbranch6
Resolution: | Keywords: r.watershed
Platform: Linux | Cpu: x86-32
----------------------+-----------------------------------------------------
Comment (by dylan):
Hmm.. It looks like this is not an overflow, rather some areas are
randomly getting their sign flipped. ABS(flow_acc) results in a map that
*looks* correct.
#518: negative flow accumulation with r.watershed SFD or MFD
--------------------------+-------------------------------------------------
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
--------------------------+-------------------------------------------------
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.
On Fri, Mar 6, 2009 at 12:18 AM, GRASS GIS <trac@osgeo.org> wrote:
#518: negative flow accumulation with r.watershed SFD or MFD
--------------------------+-------------------------------------------------
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
--------------------------+-------------------------------------------------
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!
#518: negative flow accumulation with r.watershed SFD or MFD
--------------------------+-------------------------------------------------
Reporter: dylan | Owner: grass-dev@lists.osgeo.org
Type: enhancement | Status: new
Priority: major | Milestone:
Component: Raster | Version: svn-develbranch6
Resolution: | Keywords: r.watershed
Platform: All | Cpu: x86-32
--------------------------+-------------------------------------------------
Changes (by mmetz):
* platform: Linux => All
* component: default => Raster
Comment:
added -a flag to make all flow accumulation values positive in trunk
r36273 and devbr6 r36274
What about the RUSLE factors? IMHO, they should be set to NULL when flow
accumulation could not be properly calculated (negative flow accumulation
values).