[GRASSLIST:1624] Re: GRASS 5.7: Vector networking tutorial

[Greg Sepesi]

Greg Sepesi wrote:
> 
> >
> > 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

Oops, my mistake.  Orange is higher cost than blue.  So orange isolated
in blue is okay.  (Blue isolated in orange wouldn't be.)

Greg