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

[Jarek Jasiewicz]

On 02/15/2012 03:33 PM, GRASS GIS wrote:
> #1577: r.stream.extract/r.stream.order - "calculated" mainchannel vs. "real"
> mainchannel
> -----------------------------+----------------------------------------------
>   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           |
> -----------------------------+----------------------------------------------
>   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.
what does it means? Any reference? Main channel established in real 
network or algorithms of modeling main channel?
>   an example screenshot of river basin in the eastern part of the alps is
>   attached
where it is?

Sorry. I cannot relate to this ticket, because, it is not clear what 
kind of issue it reports.

r.stream.order does not calculate "main channel". It is impossible, 
bacuase main channel is marked according different criteria, depending 
on national surveys. R.stream.order in Gravielius/Hack hierarchy (Hack 
is used after jGrass, this name is not especially correct) calculate 
stream of first order as longest channel or optionally if you use SFD it 
can use alternatively channel draining biggest basin area. That all. if 
it is main or not main channel it is interpretation. Data generated by 
hydrological modeling are deterministic realization of clear algorithms 
on the specific dataset and does not reflect reality in 100%.

Hope it helps
>   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
>
>
>
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