[GRASS-dev] [GRASS GIS] #2719: r.watershed + D8 (unexpected results)

[GRASS GIS]

#2719: r.watershed + D8 (unexpected results)
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  Reporter:  180875  |      Owner:  grass-dev@…
      Type:  defect  |     Status:  new
  Priority:  normal  |  Milestone:  7.0.3
 Component:  Raster  |    Version:  svn-trunk
Resolution:          |   Keywords:  r.watershed
       CPU:  x86-64  |   Platform:  Linux
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Comment (by mlennert):

 Replying to [comment:3 mmetz]:
 > Replying to [ticket:2719 180875]:
 > > I further found some deviations in the flow direction grid. In some
 cases the flow routing leads to the lowest neighbouring cell which however
 might not be the steepest decent. r.watershed chooses sometimes a diagonal
 flow direction while the local channel gradient would be greater in
 horizontal direction.
 > > I started some test runs with very simple geometries. I've defined a
 ramp with r.mapcalc that dips in north direction and I expected that this
 will also be shown by the flow direction of r.watershed. However, this is
 not the case. The synthetic DEM features a watershed!!! See attachment.
 It seems that r.watershed fills the region (without need). This strange
 behavior disappears by applying the "multiple flow direction" algorithm.
 > >
 > > I have tested with the latest versions grass70 , grass71 and grass645.
 > > Here is a test simple test case (grass71) that documents the
 misbehavior of r.watershed and D8.
 > >
 > > {{{
 > > g.region w=0 e=1000 s=0 n=1000 res=1 -p
 > > r.mapcalc "DEM = 1000 - y()"
 > > r.watershed -s elevation=DEM at PERMANENT accumulation=ACC drainage=DIR
 > > }}}
 >
 > The correction for diagonal flow bias missed two special cases. Fixed in
 trunk r67189, please test.

 I tested with above test case and the results now seem coherent (i.e.
 homogenous direction and accumulation from top to bottom).

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Ticket URL: <https://trac.osgeo.org/grass/ticket/2719#comment:4>
GRASS GIS <https://grass.osgeo.org>