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.
Your "physical" (i.e. on disk in GRASS format) representation of
the network(s) does not necessarily have to be the same as the
semantical (content-oriented) representation. So you can represent
the subnetworks through attribute info. Or you can play with
layers: the entire river network with the same cat value in layer
1, different cat values for subnetworks in layer 2.
Either with layers or with additonal attributes both can be solutions
but the problem is still the "how", the problem to define this
subnetworks (see picture in former posts)
/johannes
If you need to extract only parts of the network, almost all of the
modules allow you to either use cat values or a where clause on
your attribute as a filter.
Moritz
_______________________________________________
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user
_______________________________________________
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user
