[R-sig-Geo] Identifying overlapping polygons

[White.Denis at epamail.epa.gov]

r-sig-geo-bounces at stat.math.ethz.ch wrote on 2007-02-09 13:27:13:

> Andrew,
>
> Util the functionality is programmed in, here is a quick approximation
> to what you want:
>
> 1. generate a large number of random (or gridded) points within the
> polygon of interest in the first object (sample.polygon function in
> package sp).
> 2. use the overlay function in the sp package to find out which
polygons
> in object 2 the random points fall into.  You know these overlap.
>
> If there is a polygon in object 1 that only slightly overlaps a
polygon
> in object 2, then you are unlikely to detect that using this method,
but
> your statement about controlling the "Partial" indicates that this may
> be a good thing.
>
> Turning this around and generating the points from object 2 and seeing
> which polygon in object 1 they are in will tell you if the polygon in
> object 2 is completely overlapped by the one in 1.

An interesting suggestion, emulating the old manual way of computing the
area of a polygon, if you were to use a grid of points whose surrounding
areas (Voronoi polygons) were all identical.  To deal with "partial",
choose the spacing of points (hence areas of Voronoi polygons) to be a
compromise between a desired resolution and computation time, depending
on how many of these comparisons you need to make.

>
> Hope this helps,
>
> --
> Gregory (Greg) L. Snow Ph.D.
> Statistical Data Center
> Intermountain Healthcare
> greg.snow at intermountainmail.org
> (801) 408-8111
>
>
>
> > -----Original Message-----
> > From: r-sig-geo-bounces at stat.math.ethz.ch
> > [mailto:r-sig-geo-bounces at stat.math.ethz.ch] On Behalf Of Roger
Bivand
> > Sent: Friday, February 09, 2007 12:56 PM
> > To: Andrew Niccolai
> > Cc: r-sig-geo at stat.math.ethz.ch
> > Subject: Re: [R-sig-Geo] Identifying overlapping polygons
> >
> > On Fri, 9 Feb 2007, Andrew Niccolai wrote:
> >
> > > Hi.  I am relatively new to Spatial-R.  I have two objects that
are
> > > both SpatialPolygonsDataFrame classes.  One is comprised of
> > polygons
> > > from a section of a segmented image and the other is essentially
> > > circular polygons representing the potential areas that the
> > crowns of
> > > known tree locations might cover.  I would like to identify every
> > > polygon from the first object (image segmentation polys) that
> > > completely are even partially (hopefully I can control the
> > "partial"
> > > threshold) falls within any given polygon from the second object
> > > (crown areas).  In ArcGIS this can be accomplished using
> > the Identity
> > > application under ArcToolbox.  I was hoping to accomplish
> > the same feats in R.  Any help would be greatly appreciated!!
> >
> > This is not present yet, but not impossible. If you look in
> > the code of the unionSpatialPolygons() function in the
> > maptools package, you will see that it uses functions in the
> > gpclib package. Using similar ideas to examine either all n
> > by m intersections, or a clever subset by looking at bounding
> > box intersections first, it ought to be feasible. You'll need
> > to be prepared to code it, though.
> >
> > Roger
> >
> > >
> > > Andrew Niccolai
> > > Doctoral Candidate
> > > Yale School of Forestry
> > >
> > > _______________________________________________
> > > R-sig-Geo mailing list
> > > R-sig-Geo at stat.math.ethz.ch
> > > https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> > >
> >
> > --
> > Roger Bivand
> > Economic Geography Section, Department of Economics,
> > Norwegian School of Economics and Business Administration,
> > Helleveien 30, N-5045 Bergen, Norway. voice: +47 55 95 93 55;
> > fax +47 55 95 95 43
> > e-mail: Roger.Bivand at nhh.no
> >
> > _______________________________________________
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> >
>
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