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

[Joseph Guillaume]

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 v1.2tutorial.
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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