[GRASS-user] network analysis: cost columns and v.net.alloc problem

Ricardo Filipe Soares Garcia da ricardo.garcia.silva at gmail.com
Wed Jul 1 05:03:43 EDT 2009

Hello list
I have a problem with the setting up of a vector map for network analysis:

- I have two cost columns associated with the cost of traveling on
each direction of arcs (so the afcol and abcol).
- It happens that on some arcs (representing streets) sometimes it is
not allowed to travel in one direction, and in that case the cost is
set to -1
- Now I have a set of points that I can connect to the network and
then I want to calculate sub-nets for each point (using v.net.alloc)
- When I connect the points to the network (using v.net), new lines
get created to expand the network and link it with the points
- These new lines get created and are assigned the cat of the main
line from which they got extruded
- The problem is: it happens that the cost associated with the main
arc gets assigned also to this new line that connects the point to the
network. This is due to the fact that both lines share the same cat
(the key to the attribute table)
- So if the main line had a cost of -1 assigned to one of the travel
directions it means the new line will also have this same cost. This
will make it impossible to travel (to or from, according to the
affected column) to the point in the end of this line. This fact will
cause the v.net.alloc algorithm to not assign a subnet to this point,
since it is in fact inaccessible.

- Since both the main arc (the one belonging to the actual street) and
the sub-arc (the one that just connects the point to the network)
share the same cat, and thus the same entry in the attributes table,
how can I fix the costs on the sub-arc without affecting the main one?
- The only way I managed to uniquely reference these sub-arcs was with
their internal ID (which I can see when I interactively query the
vector's properties), but I can't seem to do much with it

So, if I'm still making sense, can someone please offer some advice on
how to solve this?

- Will I have to manually delete the sub-arcs and then digitize them
myself? It could work on this case, where I have only 9 points to
connect, but if I had more...
Thanks for your help

___________________________ ___ __
Ricardo Garcia Silva

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