Identifying overlapping polygons

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

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

in object 2, then you are unlikely to detect that using this method,

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.

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

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

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

--
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