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

Fri Oct 13 06:17:09 PDT 2017

Hi Johannes,

Sorry for the delayed response.

You could try v.net and then use igraph.

See: https://github.com/NINAnor/gudbrand_hydro/blob/master/v.igraph.order.py
as an example for network analysis using a GRASS python script in combination with igraph.

What you would need is to compute the paths between all possible combinations of sampling sites (using e.g. http://igraph.org/python/doc/igraph.GraphBase-class.html#get_shortest_paths).
Once you got a unique list of edges you can use v.extract I guess to get your subnetwork…

Or if you continue with network analysis you could just use:
http://igraph.org/python/doc/igraph.GraphBase-class.html#subgraph_edges

Hope that is somehow useful…

Cheers
Stefan

From: grass-user [mailto:grass-user-bounces at lists.osgeo.org] On Behalf Of Johannes Radinger
Sent: torsdag 28. september 2017 13.04
To: Markus Metz <markus.metz.giswork at gmail.com>
Cc: GRASS user list <grass-user at lists.osgeo.org>
Subject: Re: [GRASS-user] Extract subnet from grass vector network based on selected nodes

Hi all,

I just tried two different tools that both work to extract a subnetwork that connects a set of selected nodes:

1) v.net.allpairs works fine and a subnetwork is extracted. However, this definitely needs some clean up and layer/attribute operations to get the connected tables/cats from the original network

2) v.net.steiner also works fine. However, it seems that this approach takes slightly longer than v.net.allpairs.

In general both approaches are rather slow if one wants to extract a subnetwork based on e.g. >1000 selected nodes from a very large network. For example, my initial (large) network consists of >100000 lines which makes any further analysis rather slow. Thus, I wanted to minimize the network to one that still connects my target nodes but skips parts that are not needed.
Thank you for you suggestions, anyway.
/Johannes

On Thu, Sep 28, 2017 at 10:06 AM, Markus Metz <markus.metz.giswork at gmail.com<mailto:markus.metz.giswork at gmail.com>> wrote:

On Thu, Sep 28, 2017 at 9:43 AM, Moritz Lennert <mlennert at club.worldonline.be<mailto: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> <mailto: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>> <mailto: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>> <mailto: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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