[GRASS-dev] [GRASS GIS] #1840: r.drain manual example not working

[GRASS GIS]

#1840: r.drain manual example not working
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 Reporter:  neteler      |       Owner:  grass-dev@…              
     Type:  defect       |      Status:  new                      
 Priority:  major        |   Milestone:  6.4.3                    
Component:  Raster       |     Version:  svn-releasebranch64      
 Keywords:  r.drain      |    Platform:  Unspecified              
      Cpu:  Unspecified  |  
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Comment(by mmetz):

 Replying to [ticket:1840 neteler]:
 > I have added the test case from the manual in r54356 in
 > grass70/raster/r.drain/tests/
 >
 > While the manual tells that a raster line will be produced,
 > only the start pixel is found. Failure?

 The example elevation surface comes from r.drain in GRASS 3.2, or earlier.
 r.drain has been changed quite a lot since then, mainly because there were
 problems with the drainage directions determined by r.drain.

 A first attempt to solve this problem was to clone code from another
 module (r.fill.dir) in order to cope with flat areas and sinks. IMHO, the
 idea was right, but the implementation was wrong: it would have been much
 easier to simply allow a direction map as input for r.drain. As a side
 effect of this modification, an efficient mechanism to cache input data
 has been replaced with a less efficient mechanism. This change which
 happened somewhere between 4.2 and 5.4 is the reason why the example as in
 3.2 is no longer working.

 Another problem with path directions was that r.drain was not compatible
 with the Knight's move option of r.cost and r.walk. Therefore a new option
 to read a direction raster map was added to r.drain, which was actually
 the solution to all previous problems of r.drain, in theory. Raster maps
 encoding directions are already produced by some other modules, e.g.
 r.watershed (also existing since 3.2 or earlier), and the possibility to
 feed these direction maps to r.drain would have produced either least cost
 paths from r.cost/r.walk output or hydrological paths from e.g.
 r.watershed output. Unfortunately, the encoding of directions in the
 output of r.cost and r.walk is unique within GRASS and not compatible with
 GRASS standard directions using degrees CCW from East. That has been fixed
 in r54419.

 IMHO, the cleanest solution would be to revert r.drain back to the version
 of GRASS 3.2 (the earliest I have), add support for NULL cells and fp
 input, and patch in the optional direction input option indir. IOW, the
 task of determining reasonable path directions should be solved by other
 modules, not r.drain, because the calculation of these path directions is
 rather complex, see r.cost/r.walk/r.watershed/r.terraflow/r.flow. The
 manual would need to emphasize that the use of a direction input map is
 strongly recommended.

 Markus M

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Ticket URL: <http://trac.osgeo.org/grass/ticket/1840#comment:3>
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