[GRASS-dev] [GRASS GIS] #1924: r.watershed - empty stream segment map

[GRASS GIS]

#1924: r.watershed - empty stream segment map
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 Reporter:  jradinger  |       Owner:  grass-dev@…              
     Type:  defect     |      Status:  new                      
 Priority:  normal     |   Milestone:                           
Component:  Raster     |     Version:  svn-trunk                
 Keywords:             |    Platform:  Linux                    
      Cpu:  x86-32     |  
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Comment(by mmetz):

 Replying to [comment:2 jradinger]:
 >
 > Two more important things:
 > 1) The elevation input map is accutally already a thinned river raster.
 The raster
 > values represent a pseude elevation which  was generated with r.cost
 from the source.
 > Thus the value (elevation) is increasing upstream. r.watershed is just
 used for getting
 > the flowdirection and segmenting the river raster. (if needed I can
 provide the elevation-rcost
 > raster)

 r.watershed expects a surface as input. Using a thinned river raster as
 input should give weird results: all non-NULL cells face at least one NULL
 cell treated as unknown elevation, thus flow direction can not be
 unambiguously determined. You can try to place a buffer around the rivers
 and fill the buffer with pseudo-elevation values larger than the largest
 cost value. For example, create a buffer using 300 meter (3 cells) as
 distance, set a pseudo elevation value for the buffer zone, patch the
 thinned river raster with the pseudo elevation, run r.watershed with
 threshold=3. Works for me.

 > And maybe a warning/error message should be printed when threshold = 0,
 instead of the (for me confusing error)

 Makes sense.

 Markus M

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Ticket URL: <http://trac.osgeo.org/grass/ticket/1924#comment:3>
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