[GRASS-dev] [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:  critical  |   Milestone:  6.4.0                    
Component:  Raster    |     Version:  6.4.0 RCs                
 Keywords:            |    Platform:  All                      
      Cpu:  All       |  
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 When using r.watershed on a low relief smooth dem I realised that the flow
 direction surface was almost 20 times more likely to go to one of the
 diagonal pixels rather then the edge pixels. I’m sure this is not right as
 I interpret the description in Ehlschlaeger (1989) this shouldn’t happen.

 The defect is that r.watershed isn’t correcting for the larger distance
 between cells that share a corner then cells that share an edge. The
 result is that r.watershed is directing flow to the lowest cell not the
 steepest cell.

 Ehlschlaeger (1989) in the last paragraph of section 4 describes how the
 search algorithm should be adjusted to account for diagonal cells being
 further away so that flow follows the steepest path. see
 [http://chuck.ehlschlaeger.info/older/IGIS/paper.html]

 In
 [http://trac.osgeo.org/grass/browser/grass/trunk/raster/r.watershed/ram/do_astar.c#L232]
 there is code in get_slope that accounts for the diagonal but it doesn’t
 seem to be called.

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Ticket URL: <http://trac.osgeo.org/grass/ticket/807>
GRASS GIS <http://grass.osgeo.org>