[R-sig-Geo] R-sig-Geo Digest, Vol 43, Issue 14

[Tim Keitt]

Hi Nic,

The convex hull would be fast and easy to compute (there's existing
code in R). I want the ordinary hull which is the set of arcs forming
the perimeters (inside and out). My crude and very slow solution was
to convert all the polygons (in this case hexagons on a lattice) into
their constituent arcs and then for each arc count how many times it
occurs in the set (requires slightly fuzzy matching of points). Arcs
that occur more than once are removed. The remaining arcs form the
hull. Runs in about 20 minutes with a  few hundred hexagons.
Sufficient for the moment.

THK

On 3/16/07, Nicholas Lewin-Koh <nikko at hailmail.net> wrote:
> Hi Tim,
> I am not quite sure what you are getting at here. Do you want to
> intersect
> polygons and then select the set of lines that form the outer perimeter?
> Do you wan the convex hull of a set of polygons. I guess I have been out
> of the
> GIS world to long. It seems to me that this would be something easy to
> solve,
> just tedious iteration of the polygon coordinates and some
> triangulation.
>
> Nicholas
>
> > Date: Thu, 15 Mar 2007 10:49:23 -0500
> > From: "Tim Keitt" <tkeitt at gmail.com>
> > Subject: [R-sig-Geo] polygons to arcs?
> > To: r-sig-geo at stat.math.ethz.ch
> > Message-ID:
> >       <6262c54c0703150849qe60ab14nfef1eb3bf73dfb5d at mail.gmail.com>
> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> >
> > Is there an 'sp' function that takes a polygon as its argument and
> > returns a set of line objects corresponding to the arcs in the
> > polygon?
> >
> > Or better yet, a function that given a set of polygons, returns the
> > hull? (ie the set of singleton arcs after applying the polys to arcs
> > function)
> >
> > THK
> >
> > --
> > Timothy H. Keitt, University of Texas at Austin
> > Contact info and schedule at http://www.keittlab.org/tkeitt/
> > Reprints at http://www.keittlab.org/tkeitt/papers/
> > ODF attachment? See http://www.openoffice.org/
> >
> >
> >
>


-- 
Timothy H. Keitt, University of Texas at Austin
Contact info and schedule at http://www.keittlab.org/tkeitt/
Reprints at http://www.keittlab.org/tkeitt/papers/
ODF attachment? See http://www.openoffice.org/