[GRASS-dev] [GRASS GIS] #2036: Failed watershed analysis on Grass

[GRASS GIS]

#2036: Failed watershed analysis on Grass
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  Reporter:  mehmeto      |       Owner:  grass-dev@…              
      Type:  defect       |      Status:  closed                   
  Priority:  normal       |   Milestone:  6.4.4                    
 Component:  Raster       |     Version:  6.4.2                    
Resolution:  fixed        |    Keywords:  LFS, r.watershed         
  Platform:  MSWindows 7  |         Cpu:  x86-64                   
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Comment(by mehmeto):

 Replying to [comment:7 hamish]:
 > this isn't really that large of a region, you usually hit 2 GB files at
 ~ 45000x45000 rows x cols. this is much smaller,

 > > If you calculate 4 * 60175190 bytes = 240700760 which is > 2^31.
 >
 > missed a 0, actually it's 9 times smaller than 2^31. It only wants to
 allocate 241mb RAM, so probably not a large-file problem. How much do
 memory does your computer have?

 It has 8GB.

 Also thank you for updating the tutorial.

 Now I can do watershed analysis at least on a demo map but my real data
 will be larger. I will need to work on an area of about 5,000 square km.
 and the map scale will be about 1:25,000 or even lower. I will also need
 to transform contours to DEM before start using r.watershed. Since I will
 work on a PC, memory problems seem inevitable.

 I know one suggested method to overcome memory limitations is dividing a
 large map to smaller pieces but in a wathershed analysis that will mean
 false or missing results. Or, maybe I will need to divide the map into
 overlapping sections to make sure no basins are missed and then manually
 stitch the sections, which sounds to me a very laborous procedure now. Do
 you have any ideas how to do that, or any forums where I can ask?

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Ticket URL: <http://trac.osgeo.org/grass/ticket/2036#comment:13>
GRASS GIS <http://grass.osgeo.org>