[GRASSLIST:1623] Re: GRASS 5.7: Vector networking tutorial

[Greg Sepesi]

> 
> v.net.iso calculates costs from each starting point to all network nodes.
> Obviously an algorithm implemented in dglib would be much faster,
> unfortunately it is not, and dglib is difficult to change.
> 
> Radim
> 

Hi Radim,

Upon completion of the shortest path algorithm, each node in the graph
is associated with a cost to get there from the starting point.  With K
starting points within a graph with N nodes, v.net.iso consists of
running the shortest path algorithm K times, and for each of the N nodes
calculating the minimum of the K costs.  Of course the shortest path
algorithm should be performed within the directed graph library.  But it
seems the per node calculation of the minimum cost could be performed
either inside or outside the directed graph library.  I didn't put it in
the dgtable library, but it would be easy to do so.  Either way, I think
that for small K (e.g., 10) and moderate N (e.g., 10,000), the
calculation of v.net.iso can be virtually instantaneous.

As an aside, I noticed something peculiar in the v.net.iso screen dump
at

	http://mpa.itc.it/radim/g51/v.net.iso.png

At the 6th starting point from the top, there are a couple orange paths
completely isolated by blue.  Shouldn't they be connected to other
orange?

Greg