[GRASS-user] Re: voronoi for polygons

[Martin Wegmann]

Hello Martin R., 

that's great, now I understand your concept. 
I was locked in the "v.to.points" and "v.voronoi" idea and did not see the 
raster way. 

regards, Martin

On Thursday 10 August 2006 19:13, Martin Rutzinger wrote:
> hi martin w.!
>
> these steps...
>
> - convert the selected polygons to raster (v.to.rast)
> - switch null() values to 1 and pixels with values to null() (r.mapcalc)
> - thin raster (r.thin) several iterations
> - convert thinned raster to vector and make polygons (r.to.vect,...)
>
> ...derive voronoi polygons from your 'input polygons' directly so that you
> need not to "group all voronoi polygons which belong to points of one
> polygon"
>
> see:
> http://geo4.uibk.ac.at/users/rutzinger/download/topology5_grass_fire_poly.j
>pg
>
> if you have done that, you have to assign the cat-values of your 'input
> polygons to the 'voronoi polygons'. i think this should be possible with
> v.distance, but i'm not sure.
>
> hope this helps
> martin r.
>
> Zitat von Martin Wegmann <wegmann at biozentrum.uni-wuerzburg.de>:
> > Hello Martin,
> >
> > On Wednesday 09 August 2006 17:22, Martin Rutzinger wrote:
> > > i'm not sure what is wrong with v.select, but i can show a second
> > > approach how to derive voronoi polygons (restriction: doesn't work for
> > > adjacent polygons):
> >
> > thanks for your help, I tried your approach, but having polygons as input
> > (not
> > adjacent ones) and using them as base for voronoi  polygons between
> > polygons,
> >
> > this approach does not work for me or I misunderstood you:
> > > - convert the selected polygons to raster (v.to.rast)
> >
> > ok
> >
> > > - switch null() values to 1 and pixels with values to null()
> > > (r.mapcalc)
> >
> > it works for my base polygons, because they are not adjacent, but not for
> > my voronoi polygons. I have no null() values because my voronoi polygons
> > are adjacent and cover the whole location
> >
> > > - thin raster (r.thin) several iterations
> >
> > that does not work due to the previous problem, but how do you
> > distinguish between voronoi polygons belonging to the same base polygon
> > and vornoi polygons belonging to the base polygons in the neighbourhood?
> > Extracting the border between two voronoi polygon groups (for 2 base
> > polygons) would be interesting, but I have no idea how to tell GRASS
> > which line that is.
> >
> > > - convert thinned raster to vector and make polygons (r.to.vect,...)
> >
> > regards, Martin
> >
> > > this works for me
> > > /martin
> > >
> > >
> > > ********************
> > > Hello,
> > >
> > > I want to calculate voronoi areas for polygons, but I am stuck in
> > > the "query-part".
> > >
> > > using spearfish data I did:
> > >
> > > v.extract input=fields at PERMANENT output=fields_extract
> > > list=41,36,46,21
> > >
> > > v.to.points input=fields_extract output=fields_extract_points
> > > type=boundary
> > >
> > > v.voronoi input=fields_extract_points
> > > output=fields_extract_points_voron
> > >
> > >
> > > now I wanted to
> > > - group all voronoi polygons which belong to points of one polygon (GIS
> > > query overlap)
> > > - calculate area of merged voronoi polygons and assign them to each
> > > original polygon
> > >
> > > but:
> > >
> > > v.select operator=overlap gives:
> > >
> > > v.select binput=fields_extract_points_voron_clean
> > > ainput=fields_extract_clean atype=area btype=area output=test
> > > operator=overlap
> > > Processing ainput areas ...
> > >  100%
> > > Writing attributes ...
> > > Layer 1
> > > v.select complete.
> > > Building topology ...
> > > 0 primitives registered
> > > 0 areas built
> > > 0 isles built
> > > Attaching islands:
> > > Attaching centroids: Topology was built.
> > > Number of nodes     :   0
> > > Number of primitives:   0
> > > Number of points    :   0
> > > Number of lines     :   0
> > > Number of boundaries:   0
> > > Number of centroids :   0
> > > Number of areas     :   0
> > > Number of isles     :   0
> > >
> > > anything wrong with the command?
> > > I did it as well without b/atype=area but same result.
> > >
> > > The problem how to calculate areas is not yet tackled ,-)
> > >
> > > I appreciate any hint, Martin