[GRASS-user] v.net.salesman unreachable nodes

[Markus Neteler]

Joseph,

it is most likely that your nodes aren't physically connected to the
network, or that they are connected but have no category number
or the conneting line doesn't have a category number. Myself, I wasted
much time with such problems. But this is history now :)

Since recently, GRASS 6.3 contains an updated v.net (thanks to Martin
Landa) which makes it a snap to connect nodes to a graph ("connect" tool).

I have just added a v.net.steiner example:
http://grass.itc.it/grass63/manuals/html63_user/v.net.steiner.html

Please take a look - it's very easy now (before this update of v.net,
connecting nodes was a nightmare). v.net.salesman should work in
a similar way.

Markus

Joseph Guillaume wrote on 08/03/2007 10:37 AM:
> Hi,
>
> I'm trying to run a travelling salesman analysis between point
> features on a road map, but it tells me that two nodes are unreachable
> from one another, which is a fatal error.
>
> I've successfully done this exact analysis using ARC/View, but I'm
> trying to find an OSS alternative that works on Debian - and it seems
> GRASS is the only software currently able to do network analyses.
>
> I patched together two ESRI shapefile datasets, following the Grass
> 6.0 v1.2 tutorial.
> I tried linking the attributes to the original datasets and tried
> without linking.
> I tried putting the new points on both the same layer and a new layer.
> I tried running every single v.clean function on both the original and
> patched datasets.
> I am certain that all the points are actually on the network.
> I've searched all mailing lists, reference materials, and bugtracker,
> but didn't find anything explicitly related to unreachable nodes.
>
> Assuming the error message does mean that the map I am using has
> segments disconnected from the rest of the network, the questions are:
>
> * How do I detect them and remove them?
> The node numbers given don't match the cat attribute, so I assumed
> they referred to the id attribute. The two nodes referred to by id are
> however connected to each other (directly) and to the main part of the
> network. I suspect the node numbers therefore don't correspond to any
> attribute as such, but are rather internal. In that case, how do I
> identify the actual nodes/segments responsible?
>
> * Shouldn't the algorithm be tolerant of such errors anyway?
> I checked, and none of the relevant nodes are actually within segments
> I can see are disconnected, so the analysis results should be the same
> if the algorithm ignored the error... Is this perhaps because of the
> heuristics used to solve this NP-hard problem? Does anyone know what
> ArcView does differently?
>
>
>
> Any ideas?
>
> Cheers,
>
> Joseph Guillaume


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ITC -> dall'1 marzo 2007 Fondazione Bruno Kessler
ITC -> since 1 March 2007 Fondazione Bruno Kessler
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