[GRASS-dev] Re: [GRASS GIS] #807: r.watershed doesnt consider longer distance to diagonal neighbouring pixels

[GRASS GIS]

#807: r.watershed doesnt consider longer distance to diagonal neighbouring pixels
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  Reporter:  aread   |       Owner:  grass-dev at lists.osgeo.org
      Type:  defect  |      Status:  new                      
  Priority:  major   |   Milestone:  6.4.0                    
 Component:  Raster  |     Version:  6.4.0 RCs                
Resolution:          |    Keywords:  r.watershed              
  Platform:  All     |         Cpu:  All                      
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Comment (by mmetz):

 Replying to [comment:5 hamish]:
 > Recommendation: wait to backport this to the 6.4.x line until after
 6.4.0 has shipped,
 > and have it go out in the 6.4.1 release instead. otherwise we only have
 time for
 > cursory checks. Waiting for 6.4.1 buys us some time.

 Hmm, about time, nobody checked in the last 17 years or so? grass40 was
 released in 1992 I learned, and there the correction for diagonal flow
 bias was already disabled. It was probably not related to lat/lon
 projections because 1) I'm not sure if grass supported lat/lon back then,
 2) there is a lot of slope calculation for the RUSLE factors that doesn't
 care about lat/lon locations and is still active. Making r.watershed
 lat/lon aware is (another) major rewrite... It was more probably related
 to a severe speed penalty imposed by the original implementation, avoided
 in the new implementation (which is also more in line with A* Search
 logic). It is really a pity that the commit comments from grass32 to
 grass40 are no longer available. I also did not find anything in the
 mailing list archives, although r.watershed was apparently regularly used.

 Hydrologists at CSIRO in Australia are currently looking at r.watershed in
 depth and tear the algorithm apart, looking at all sorts of special cases.
 So far the corrected version produces pretty much optimal results.

 > We could add a BUGS section to the 6.4.0 help page if needed.

 BTW, there is another bug in SECTION 4: Watershed determination, I'm busy
 fixing it.

 Markus M

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Ticket URL: <https://trac.osgeo.org/grass/ticket/807#comment:6>
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