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

Fri Mar 16 19:11:19 CET 2007

Denis,

I was just about to reply to Nicholas that my ideal solution would be
to build a graph data structure linking adjacent elements and find the
the hull that way -- I think you saw the solution before I did. (I'm
starting to grok that all of this is already well known in the vector
GIS world where topology has been thoroughly studied.)

I'd be happy to discuss with folks the idea of using the Boost Graph
Library to implement topological relationships for spatial objects.
We've already produced code that links the BGL to GDAL -- specify an
adjacency rule and then run eg cost path analysis on GDAL rasters
(look for this on sourceforge soon). It would also be possible to
implement something similar with vector objects.

My OSGeo summer of code idea is to link the BGL to postgis. I wasn't
originally thinking about topology per se, but it would achieve much
the same thing.

If people have ideas about how to fund these sorts of things, I'd be
happy to hear about it. The implementation end is rather far from my
core research. Its mostly been side projects.

THK

On 3/16/07, White.Denis at epamail.epa.gov <White.Denis at epamail.epa.gov> wrote:
> 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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>
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-- 
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/