# [R-sig-Geo] agenda for R's Geo capabilities?

White.Denis at epamail.epa.gov White.Denis at epamail.epa.gov
Fri Mar 16 17:06:21 CET 2007

```The thread below and many others in R-sig-Geo raise questions about
future directions.  In reinventing GIS there are a whole list of
capabilities and functions that would be helpful.  Some that I have
noticed include,

Topological representation to enable
Planar enforcement of boundary integrity of polygon tessellations
"Dissolving" interior edges easily as in the thread below
Large problem computational geometry functions
Identify many points inside of many polygons
Intersections/overlays of two sets of many polygons
Distances between all pairs of many polygons

Are there members of the R Geo community working on any of these?
Are these issues seen as an exclusive focus of commercial GIS?
Are there discussions about these issues at relevant conferences?

(I will be at AAG in San Francisco and would be happy to meet with
others if there is interest.)

r-sig-geo-bounces at stat.math.ethz.ch wrote on 2007-03-16 07:52:59:

> 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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