[GRASS-user] Tessellation of a set of polygons

[Thomas Leduc]

Dear Markus,

Impressive responsiveness. Thanks for the dedicated development.
http://trac.osgeo.org/grass/changeset/57621 is fully operative on the given
dataset.

It took me some time to test it, because I'm a complete newby with GRASS
community. Just to keep it written somewhere, I first had to checkout,
configure, compile, and install GRASS7 snapshot on my Debian 6.0.7:

    svn checkout https://svn.osgeo.org/grass/grass/trunk grass7_trunk
    cd ./grass7_trunk
    ./configure --enable-64bit
-with-freetype-includes=/usr/include/freetype2
--prefix=/usr/local/misc/grass7-snapshot --with-sqlite
    make
    make install

Then, in the GRASS console, I've copied and pasted the following
instructions:

    g.remove vect=buildings,vor
  v.in.ogr -o dsn=/tmp/src/src.shp output=buildings
  v.voronoi -a --overwrite input=buildings output=vor segment=0.15
  v.out.ogr --overwrite input=vor type=area dsn=/tmp/src olayer=vor layer=1
format=ESRI_Shapefile

Many thanks once again.

Thomas


On Sun, Sep 8, 2013 at 7:49 PM, Markus Metz
<markus.metz.giswork at gmail.com>wrote:

> On Thu, Aug 29, 2013 at 5:38 PM, Thomas Leduc
> <thomas.leduc at cerma.archi.fr> 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.
> >
> I have added tessellation of areas to v.voronoi in trunk r57621. You
> need to use the new -a flag and can control the smoothness of the
> output with the new segment option. With your example, segment=0.15
> produces reasonably smooth output.
>
> This implementation ignores shared boundaries (not present in your
> example) which would need to be patched in afterwards. An alternative
> would be to use v.overlay to obtain areas outside the original input
> areas.
>
> HTH,
>
> Markus M
>
> > Thanks a lot for any help/advice.
> >
> > Thomas Leduc
> >
> > PS: I'm aware there already exist in GRASS some functions such as
> > "v.to.rast, r.grow.distance, and r.contour" which could help in building
> map
> > of distances, and isocontours... I'm also aware there already exist
> > "v.to.rast, r.thin, and r.to.vect feature=line" to build some sort of
> medial
> > axis transform or skeleton.
> >
> > PPS: input data set
> >
> > create table b(gid int, the_geom GEOMETRY(POLYGON, 2D));
> > insert into b values (0, GeomFromText('POLYGON ((0 0, 10 0, 10 10, 0 10,
> 0
> > 0))'));
> > insert into b values (1, GeomFromText('POLYGON ((30 0, 30 -30, 10 -30, 10
> > -40, 40 -40, 40 0, 30 0))'));
> > insert into b values (2, GeomFromText('POLYGON ((20 20, 100 20, 100 40,
> 60
> > 40, 60 30, 40 30, 40 60, 20 60, 20 20))'));
> > insert into b values (3, GeomFromText('POLYGON ((60 10, 60 -20, 100 -10,
> 100
> > 10, 60 10))'));
> > insert into b values (4, GeomFromText('POLYGON ((-10 -10, -10 -40, -40
> -40,
> > -40 -20, -60 -20, -60 0, -50 0, -50 -10, -10 -10))'));
> > insert into b values (5, GeomFromText('POLYGON ((-40 20, -40 40, -10 40,
> -10
> > 30, -20 20, -40 20))'));
> >
> > [1] this screenshot of about 11 KB is also downloadable at
> > https://dl.dropboxusercontent.com/u/8846569/tessellation.png
> >
> >
> > _______________________________________________
> > grass-user mailing list
> > grass-user at lists.osgeo.org
> > http://lists.osgeo.org/mailman/listinfo/grass-user
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-- 
Thomas LEDUC
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