[GRASSLIST:1638] Re: GRASS 5.7: Vector networking tutorial
Markus Neteler
neteler at itc.it
Sat Nov 1 05:47:49 EST 2003
On Fri, Oct 31, 2003 at 10:52:31AM -0500, Greg Sepesi wrote:
>
>
> 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,
just FYI: The vector data are here:
http://frida.intevation.org/
Markus
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