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

Nicholas Lewin-Koh nikko at hailmail.net
Fri Mar 16 15:52:59 CET 2007

```Hi Tim,
You could compute the convex hull first, and then iterate from points
on the convex hull. That should be much faster already, especially since
hexagons are convex and the perimeter will be locally convex around all
the
points touching the convex hull. You could do a variation
of the "monotone pieces" algorithm that is used in computational
geometry.
But this is a simpler problem. Are there cases with interior holes?

I have been meaning to write something like this for hexbin for a while.
There
are many cases where it would be nice to find approximations to the
density contours
and a quick and dirty way is to threshold the hexagon counts, find the
hull and
smooth the perimeter.

Nicholas

On Fri, 16 Mar 2007 08:34:20 -0500, "Tim Keitt" <tkeitt at gmail.com> said:
> 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/

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