On 25/04/11 18:37, Johannes Radinger wrote:
Am 25.04.2011 um 15:51 schrieb Moritz Lennert:
On 25/04/11 12:04, Micha Silver wrote:
On 04/24/2011 03:36 PM, Johannes Radinger wrote:
Hi Micha,
Hi other GRASS users!
Am 23.04.2011 um 18:01 schrieb Micha Silver:
So if I understand you correctly, it is not really possible to split a
river network at one point into to parts. I attached a picture to
illustrate what I want (e.g. two break points in a rivernet). I think
that this kind of breaking I want to do is quite common, so there's
probably another solution in grass gis...
Well, in QGIS you can merge all the line segments of the original
shapefile into one, using the "Vector->Geometry Tools->Singleparts to
Multipart" tool. This will leave you with a single "MULTILINE" feature.
However when importing to GRASS all the lines will be "re-broken" at
each intersection and new nodes created in order to enforce topology.
Referring to the attached image (I altered yours) the stars represent
topological "errors" and as far as I know GRASS will not allow you to
ignore these intersections when you import. (The only vector feature
that can be imported without topology is points).
So to recap:
* You can split line features at arbitrary points using v.edit
tool=break
* You cannot, AFAIK, cause GRASS to merge line segments where
topological rules require there to be a node.
You can, however, attribute the same category number (or the same
other arbitrary attribute) to those segments which belong to the same
subnetwork.
For sure that would be a solution to have the same category number for
all single segments in a subnetwork. But how to define the subnetwork
between the breakpoints, or how can I tell grass which segments belong
to a subnetwork, so far I don't know any tool in GRASS which I can use
in a python script.
GRASS cannot guess which segment belongs to which subnet. This is an
information that you have to provide GRASS with in some form.
Maybe v.strahler in the GRASS AddOns can help you. Or you can use
r.watershed to create basins and then attribute the basin numbers to
your vector lines ?
Moritz
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