On 02/15/2012 03:33 PM, GRASS GIS wrote:
#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.
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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