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

M S mseibel at gmail.com
Fri Aug 3 06:38:44 EDT 2007

If you bring the patched data (nodes/terminals and arcs) into v.digit, do
you see dangling arcs (red) that might cause the network to be
disconnected?  Particularly at the patched nodes?

On 8/3/07, Joseph Guillaume <josephguillaume at gmail.com> wrote:
> 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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