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

[Markus Metz]

trac doesn't send the new comment to the ml???

see

https://trac.osgeo.org/grass/ticket/807#comment:10


GRASS GIS wrote:
> #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
>
>