[GRASS-dev] Re: [GRASS GIS] #398: r.watershed with MFD

GRASS GIS trac at osgeo.org
Sat Dec 13 04:20:55 EST 2008


#398: r.watershed with MFD
--------------------------+-------------------------------------------------
  Reporter:  mmetz        |       Owner:  grass-dev at lists.osgeo.org
      Type:  enhancement  |      Status:  new                      
  Priority:  minor        |   Milestone:  7.0.0                    
 Component:  Raster       |     Version:  svn-trunk                
Resolution:               |    Keywords:                           
  Platform:  Unspecified  |         Cpu:  Unspecified              
--------------------------+-------------------------------------------------
Comment (by mmetz):

 Replying to [comment:2 hamish]:
 > Replying to [ticket:398 mmetz]:
 > > With the attached patch I want to submit the last (for the time being)
 modifications to r.watershed. This patch fixes a bug in flag
 initialization, also present in the original (pre-MFD and pre-faster-
 search) version.
 >
 > could you isolate that bugfix, i.e. what needs to be backported to 6.4?
 >
 Attached as r.watershed.bugfix.grass64.patch
 >
 > > Flow accumulation with SFD (single flow direction) is unchanged
 > > apart from flow accumulation output now being DCELL instead of
 > > CELL.
 >
 > nice.
 >
 Also the code for SFD is unchanged apart from flow accumulation output now
 being DCELL instead of CELL, maybe I have to emphasize that.

 > > Flow accumulation with MFD (multiple flow direction) is supported
 > > and enabled by default to keep the user interface similar to
 > > r.terraflow.
 >
 > ok, if there is a major change like that in grass7 it is worth
 > noting clearly in the help page. also the help page needs updating
 > to reflect that it is no longer "much slower than r.terraflow"

 Done in attached patch r.watershed.manpage.patch.

 > the parts referring to the A^T^ search algorithm, and any references.
 > I would guess to leave Ehlschlaeger's paper there for the
 > r.watershed ref even if the search algorithm is replaced.
 >
 The A^T^ search algorithm is untouched and in place and also used for MFD.
 I have already included a reference for MFD, but the actual implementation
 is a hybrid of MFD and the flow path determined with the A^T^ search
 algorithm. The A^T^ path, the result of the A^T^ search, is the guidance
 around which flow is distributed in MFD. Further on, MFD can't get out of
 depressions and over obstacles, but the A^T^ path can and still does in
 MFD. Chuck Ehlschlaeger's paper must stay because also with MFD the whole
 concept of r.watershed is based on Chuck Ehlschlaeger's paper. The only
 part that has been replaced is the sorting method for the A^T^ search to
 speed up the analysis, the A^T^ search itself works as before. For MFD, I
 have not replaced anything, I have only added code, ok, modified existing
 code to switch between SFD and MFD, but nothing else.

 The attached patch named r.watershed.manpage.patch updates the manpage
 with your suggestions if not already present.

 Markus

-- 
Ticket URL: <http://trac.osgeo.org/grass/ticket/398#comment:3>
GRASS GIS <http://grass.osgeo.org>


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