[GRASS-dev] [GRASS GIS] #1577: r.stream.extract/r.stream.order - "calculated" mainchannel vs. "real" mainchannel

[GRASS GIS]

#1577: r.stream.extract/r.stream.order - "calculated" mainchannel vs. "real"
mainchannel
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 Reporter:  hellik           |       Owner:  grass-dev@…              
     Type:  enhancement      |      Status:  new                      
 Priority:  minor            |   Milestone:  6.4.2                    
Component:  Raster           |     Version:  svn-releasebranch64      
 Keywords:  wingrass, addon  |    Platform:  MSWindows Vista          
      Cpu:  x86-32           |  
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 hi,

 tested with wingrass64svn, srtm v4.1 (lat/long, ETRS89-LAEA, UTM zone N33)
 and a river basin in the eastern part of the
 alps (if needed,a test-location is available).

 there'a really nice grass-addon available (thanks to Margherita) which
 calculates some
 ecological relevant characteristics of river-basins:

 http://trac.osgeo.org/grass/browser/grass-
 addons/grass6/raster/r.basin/r.basin.py

 the mainchannel of such a river basin is calculated by some calculation
 steps of r.stream.extract [1]
 and r.stream.order [2]

 http://trac.osgeo.org/grass/browser/grass-
 addons/grass6/raster/r.basin/r.basin.py#L261
 http://trac.osgeo.org/grass/browser/grass-
 addons/grass6/raster/r.basin/r.basin.py#L368

 there seems to be sometimes a difference between the "calculated" and the
 "real" (in the meaning of official hydrography,
 science literature, etc.) mainchannel.

 an example screenshot of river basin in the eastern part of the alps is
 attached

 may I cite Margherita:

 {{{
 1) stream extraction, by r.stream.extract. This step is dependent by the
 threshold.
 2) stream ordering. The Hack order assigns the value 1 to the longest
 contributor, so that it actually individuates the main channel. This is
 done by r.stream.order. Successively, r.basin just does a series of monkey
 steps to turn the first hack order segment into vector and query the
 length. When Jarek wrote r.stream.order, i asked him to add Hack order
 just
 for that. When i was testing it, sometimes i raised with him the problem
 that you mention, and i don't remember if either he fixed it or wasn't
 able
 to reproduce the issue.
 Said that, i would suggest to raise the issue in dev-ML, so that either
 Markus Metz (from the r.stream.extract side) or Jarek (from the
 r.stream.order side) could answer and likely fix the problem. I would also
 suggest you to verify if the problem is due to a mistake in stream
 extraction, say the individuated mainchannel   is actually longer than the
 other (natural) one (and this could be likely fixed by varying the
 threshold) or in hack ordering assignment, which is not able to
 individuate
 the longest segment. Only in this second case, a bug-fix by Jarek is
 needed.
 }}}

 any ideas how to get the calculated mainchannel matching the real
 mainchannel?
 I've tried a few different thresholds, but I've always get this
 "mismatch".

 best regards
 Helmut

 [1] http://trac.osgeo.org/grass/browser/grass-
 addons/grass6/raster/r.stream.extract
 [2] http://trac.osgeo.org/grass/browser/grass-
 addons/grass6/raster/r.stream.order

-- 
Ticket URL: <http://trac.osgeo.org/grass/ticket/1577>
GRASS GIS <http://grass.osgeo.org>