[R-sig-Geo] Identifying overlapping polygons

[Greg Snow]

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.

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
> > 
> > _______________________________________________
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> > R-sig-Geo at stat.math.ethz.ch
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> > 
> 
> --
> 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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