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Dears Jorge, Kinda, Tassos:

Thanks a lot for your feedbacks.

I have tried to solve the problem within Pajek. To make the graph unweighted and undirected is easy, all there is to do is to symmetrize the network. But the double-node's issue is unsolvable in Pajek. The .csv export doesn't work also because, as Jorge points out, there's no info about the graph, just about the segments' attributes. And to do it manually (in excel or whatever) for graphs with hundreds of thousands of nodes is, of course, impossible.

Nonetheless, Jorge's insight is particularly revealing. In Alasdair Turner's (2007) paper "From axial to road-centre lines" there's the following note (p. 5):
"Directionality is important: you must leave from the segment in the same direction as you arrived. Within our implementation this handled by incorporating 'back' links and 'forward' links for the connections at each end of the segment. If you entered via a 'back' link, the you must leave via a 'forward' link, and vice versa." This seems to be implemented not exactely like this but, like Jorge says, through the duplication of the nodes and through the introduction of a maximum weighted edge between the duplicates.

Now, the point is: to solve the problem of "one-direction-only" paths (quoting Jorge, "the graph can't allow turning back to get a more favourable angle for changing direction"), Depthmap uses a strategy that has nothing to do with the hteoretical construct of the graph itself, but with a detour to solve an implementation issue. If this is allright with normal depthmap analysis, it also makes the Pajek export for segment maps useless in my opinion (if it remains "literal", as it is now). Either the export should be a one-segment/one-node weighted graph (accordingly to the distance metric used, topological, angular or metric), or a simple one-segment/one-node unweighted, undirected graph. The first option is probably best, because the second is easily achieved in Pajek. In any case, having double nodes is meaningless from the point of view of a "bare" network analysis (i.e. outside Depthmap and within any other network analysis package).

I would recommend that, when exporting to Pajek, all double-nodes should be collapsed onto a single one. Otherwise, there's no point in exporting segment maps in .net format. That would be inestimably useful for my work, at least.

One last remark to Kinda's proviso, if topological segment graphs have any analytical interest outside the framework of standard angular analysis. I think they do, because we need to know with which structures we are dealing with, to compare them and to know what it trully entails to use one or another. One thing is to accept them theortically (which, in priciple, we all do), another is to be able to systematically deconstruct them (or to reverse-engineering them, if you like), in order to step on firm ground whenever doing any kind of "deep-mining" research in space syntax.   

Thanks again for all your contributions and for the continuous effort in making Depthmap an increasingly consistent spatial network analysis tool.

All the best,
Miguel