[GRASS-user] v.net.iso - service area

[Johannes Sommer]

Hi Richard and Daniel,

thanks for your answers. I don't think its as easy as building a convex hull around the endpoints of the networks segments.

Have a look at the picture example of v.hull [1] in the docs. A convex hull only connects the outer points of a point vector and doesn't care about "inner" points. 
The service area should be somehow a convex hull including a connection of inner points. Imagine also not only one but more seed points from which the isolines in the network are created.
I think the delaunay triangulation [2] would fit better. 
The problem here is that attributes (e.g. cat from v.net.iso) are lost when creating a delaunay triangulation from the endpoints of the network segments.
Thus I tried to intersect the network graph (from v.net.iso) with the triangles to get the values of the categories. But does not work either because the intersection returns to many triangles that only touch the end of the network segments but are not a real part of the service area.

I also tried a raster approach:
- Extract vertices from all lines of the result set from v.net.iso (3 classes)
- Interpolation between these points per Nearest Neighbor
- Conversion to polygons

But that's not really satisfying because it only comes close to the expected result.

I'm still searching to find any appropriate solution.

regards,
Johannes


[1] http://grass.itc.it/grass62/manuals/html62_user/v.hull.html
[2] http://grass.itc.it/grass62/manuals/html62_user/v.voronoi.html



-------- Original-Nachricht --------
> Datum: Tue, 06 Apr 2010 07:41:50 +1000
> Von: Richard Chirgwin <rchirgwin at ozemail.com.au>
> An: Daniel Victoria <daniel.victoria at gmail.com>
> CC: grass-user at lists.osgeo.org
> Betreff: Re: [GRASS-user] v.net.iso - service area

> Daniel - Thanks! (Remind myself to re-read the manual for the functions 
> I don't often use!)
> 
> Richard
> 
> Daniel Victoria wrote:
> > If I understand you correctly, v.hull might help to create the service
> > area polygon from the points....
> >
> > http://grass.itc.it/grass62/manuals/html62_user/v.hull.html
> >
> > On Sun, Apr 4, 2010 at 8:12 PM, Richard Chirgwin
> > <rchirgwin at ozemail.com.au> wrote:
> >   
> >>> ------------------------------
> >>>
> >>> Message: 3
> >>> Date: Sat, 03 Apr 2010 23:11:26 +0200
> >>> From: "Johannes Sommer" <Johann.online at gmx.de>
> >>> Subject: [GRASS-user] v.net.iso - service area
> >>> To: grass-user at lists.osgeo.org
> >>> Message-ID: <20100403211126.313360 at gmx.net>
> >>> Content-Type: text/plain; charset="iso-8859-1"
> >>>
> >>> Hi list,
> >>>
> >>> recently I played around with the network functionality in ArcGIS. I
> >>> generated so called "Service Areas" in a network dataset which return
> the
> >>> regions that can be accessed in a network dataset based on several
> distances
> >>> (in my example 150, 400 meters).
> >>>
> >>> I wanted to reproduce the results with GRASS GIS and after a short
> search
> >>> I found the module v.net.iso which seemed to fit my needs. It returns
> >>> exactly the same results as ArcGIS did on isolines on a network, but I
> can't
> >>> find any solution concerning the service areas.
> >>>
> >>> Can anyone point me in the right direction to calculate zones (that is
> >>> "build polygons from the end points of each network segment") from
> these
> >>> generated isolines in a network?
> >>>
> >>> Regards,
> >>>
> >>> Johannes
> >>>
> >>>
> >>>
> >>>       
> >> Johannes,
> >>
> >> I don't think there's anything to do this automatically in Grass-GIS.
> But I
> >> think you should be able to construct a script that would connect pairs
> of
> >> points.
> >>
> >> So if you can extract each end point of the network segment, you would
> end
> >> up with a vector containing only points - then create a temporary map
> of a
> >> point pair, run (say) v.distance output=connectors on each point pair,
> patch
> >> the resulting maps together ...
> >>
> >> It sounds a bit labourious, and probably someone else can think of
> something
> >> easier, but if you get all the individual steps right, a script is then
> easy
> >> to create.
> >>
> >> (Actually, Johannes, I think the idea is brilliant. I've done lots of
> >> network service area work, and it never occurred to me before!)
> >>
> >> Richard Chirgwin
> >>
> >> _______________________________________________
> >> grass-user mailing list
> >> grass-user at lists.osgeo.org
> >> http://lists.osgeo.org/mailman/listinfo/grass-user
> >>
> >>     
> >
> >   
> 
> _______________________________________________
> grass-user mailing list
> grass-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-user

-- 
GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT!
Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01