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

GRASS GIS trac at osgeo.org
Mon Feb 8 23:24:22 EST 2010


#807: r.watershed doesnt consider longer distance to diagonal neighbouring pixels
---------------------+------------------------------------------------------
  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                      
---------------------+------------------------------------------------------
Comment (by helena):

 Markus was right - the difference was due to handling elevations as int
 versus fp,
 so now I got the version with the diagonal fix
 and here is the comparison of spatial patterns

 grass64RC05 - SFD with integer elevation
 range ~ -200000 - +200000
 note zig-zag main streams, missing flow accumulation along the road on the
 west ridge
 http://skagit.meas.ncsu.edu/~helena/grasswork/accum5K_gr64rc5_3d.jpg
 http://skagit.meas.ncsu.edu/~helena/grasswork/accum5K_gr64rc5.png

 grass65 compiled in Sep 2009, SFD with FP before diagonal fix
 range ~ -200000 - +55000 (why so much lower than grass64?)
 no zig-zag on main streams, more realistic pattern on streams, lots of
 diagonals on hillslopes
 http://skagit.meas.ncsu.edu/~helena/grasswork/accum5K_gr65_sep09.jpg
 http://skagit.meas.ncsu.edu/~helena/grasswork/accum_5Ksep09.jpg

 grass65 compiled Feb 2010, SFD with fp after diagonal fix
 range only slightly different, quite different pattern on hillslopes -
 note particularly NW section of the watershed where the previously
 diagonal flow changed to horizontal
 http://skagit.meas.ncsu.edu/~helena/grasswork/accum5K_gr65_feb10c_i.jpg
 http://skagit.meas.ncsu.edu/~helena/grasswork/accum_5Kdiag2010.jpg

 I can see that somebody might have liked the diagonal biased version
 better than the correct one.

 grass65 MFD - no difference between sep 2009 and feb 2010
 range ~ -200000 - +16000 (lower than SFD as it should be, but still why
 such a diff between 64 and 65?)
 most realistic overall
 http://skagit.meas.ncsu.edu/~helena/grasswork/accum5K_gr65_mfd.jpg
 http://skagit.meas.ncsu.edu/~helena/grasswork/accum_5Kmfdi.jpg

 Compared to r.watershed in GRASS65, the GRASS64 results look really bad
 for this
 high resolution data - I assume for lower resolution data the difference
 won't be
 as stark but still grass64 will be much worse than the grass65 version.
 Should grass65 version of r.watershed be backported to grass64?
 although the difference in values needs to be explained (it may be mistake
 on my side)
 and Markus M may have some additional issues that need to be addressed,
 what do others think? who makes the decision?

 Helena

-- 
Ticket URL: <https://trac.osgeo.org/grass/ticket/807#comment:9>
GRASS GIS <http://grass.osgeo.org>


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