[GRASS-user] Tessellation of a set of polygons

[Thomas Leduc]

Dear Moritz,

Thanks a lot for your quick answer and sorry for my own "latency". Yes,
indeed, the Medial Axis Transform (or Skeleton) of Polygons dataset can be
computed via the Voronoi diagram of the corresponding set of points
(discretizing the polygons boundaries). However, I assume that this is not
a scalable solution with a huge set of polygons (insofar as the v.to.points
resolution has to be of enough fine grain). I was thinking to a
morphological thinning based solution... Maybe I'm wrong.

With GRASS GIS 6.4.0+42329, and the enclosed (small) dataset, the
tessel.shp output file does not match expectations. Could you please tell
me where I'm wrong?

g.remove vect=buildings,newbuildings,vor,tessel
 v.in.ogr dsn=/tmp/src/src.shp output=buildings
g.region vect=buildings

v.category input=buildings output=newbuildings option=add type=boundary
v.out.ogr input=newbuildings type=boundary dsn=/tmp/src olayer=build
layer=1 format=ESRI_Shapefile

v.to.points -i --overwrite input=newbuildings output=newbuildings_pts
dmax=0.1
v.out.ogr input=newbuildings_pts type=point dsn=/tmp/src olayer=pts layer=1
format=ESRI_Shapefile
 v.voronoi -l --overwrite input=newbuildings_pts output=vor
v.out.ogr input=vor type=line dsn=/tmp/src olayer=vor layer=1
format=ESRI_Shapefile

v.dissolve input=vor output=tessel

v.out.ogr input=tessel type=line dsn=/tmp/src olayer=tessel layer=1
format=ESRI_Shapefile

Thanks a lot once again for your help.

Thomas Leduc


On Mon, Sep 2, 2013 at 3:13 PM, Moritz Lennert <mlennert at club.worldonline.be
> wrote:

> On 29/08/13 17:38, Thomas Leduc wrote:
>
>> Let's consider a set of input polygons (represented by dark gray
>> polygonal footprints in the enclosed screenshot [1]). I wonder whether
>> there already exists a (set of) GRASS instructions to produce the
>> "surrounding tessellation". I mean, could you please tell me how to
>> compute the set of multicolor polygons in the enclosed screenshot?
>>
>> As an example, the blue polygon is the set of points that are:
>>    - overlayed by the dark gray polygon labeled #1
>>    - or closer to the dark gray polygon labeled #1 than to any other
>> input polygon.
>>
>> Thanks a lot for any help/advice.
>>
>
> You could try the following workflow:
>
> - add category values to your boundaries (if that is important make them
> equal to the cat values of your centroids, otherwise just using v.category
> op=add type=boundary)
> - use v.to.points to transform the boundaries to points (each point will
> have the cat value of the corresponding boundary)
> - v.voronoi to create voronoir diagrams for these points
> - v.dissolve to disolve boundaries between polyons with the same category
> numbers
>
> Moritz
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>



-- 
Thomas LEDUC
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