[GRASS-user] Extract subnet from grass vector network based on selected nodes

[Markus Metz]

On Thu, Sep 28, 2017 at 9:43 AM, Moritz Lennert <
mlennert at club.worldonline.be> wrote:
>
> On 28/09/17 08:51, Markus Metz wrote:
>>
>>
>>
>> On Wed, Sep 27, 2017 at 11:55 PM, Moritz Lennert <
mlennert at club.worldonline.be <mailto:mlennert at club.worldonline.be>> wrote:
>>  >
>>  > On 27/09/17 21:03, Markus Metz wrote:
>>  >>
>>  >>
>>  >>
>>  >> On Wed, Sep 27, 2017 at 4:07 PM, Moritz Lennert <
mlennert at club.worldonline.be <mailto:mlennert at club.worldonline.be> <mailto:
mlennert at club.worldonline.be <mailto:mlennert at club.worldonline.be>>> wrote:
>>  >>  >
>>  >>  >
>>  >>  >
>>  >>  > Le 27 septembre 2017 13:11:54 GMT+02:00, Johannes Radinger <
johannesradinger at gmail.com <mailto:johannesradinger at gmail.com> <mailto:
johannesradinger at gmail.com <mailto:johannesradinger at gmail.com>>> a écrit :
>>  >>  > >Hi,
>>  >>  > >
>>  >>  > >I have a GRASS vector network that represents a river network
(with
>>  >>  > >many
>>  >>  > >first order tributaries) and that has additional connected nodes
that
>>  >>  > >represent sampling sites.
>>  >>  > >
>>  >>  > >I'd like to extract a minimum subnetwork of the full network
that still
>>  >>  > >connects a set of selected nodes (e.g. identified by their cat).
>>  >>  > >However,
>>  >>  > >network edges (i.e. river segments) that are not necessary to
connect
>>  >>  > >the
>>  >>  > >sampling points should be excluded in the new subnetwork. Is
there a
>>  >>  > >function or process in GRASS GIS to extract such a subnetwork
that
>>  >>  > >fully
>>  >>  > >connects a set of selected nodes?
>>  >>  >
>>  >>  > not sure but maybe v.net.spanningtree ?
>>  >>
>>  >> v.net.spanningtree calculates a tree covering all nodes in the
network, not only selected nodes, therefore v.net.spanningtree does not
apply here.
>>  >
>>  >
>>  > If you connect only the selected nodes to the network, wouldn't that
work ? Or does v.net.spanningtree consider all connections between lines as
nodes ?
>>
>> v.net.spanningtree considers all internal nodes of the network. See also
>> https://en.wikipedia.org/wiki/Spanning_tree
>
>
> Ok, thanks. So, one would need to "disconnect" lines at non-selected
nodes for this to work.

or use v.net.steiner (see my previous reply)
>
> And maybe some clarification on what is meant by "nodes" in the sentence
"A spanning tree is a minimum cost subnetwork connecting all nodes in an
undirected network" in the man page might help future users.

Yes, that would help. I needed to look at the library fn
NetA_spanning_tree() to be sure.

Markus M
>
> Moritz
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