[R-sig-Geo] intersection of two polygons

[Roger Bivand]

On Tue, 20 Mar 2007, Stéphane Dray wrote:

> I have a code that do the job for polylist objects at 
> http://biomserv.univ-lyon1.fr/~dray/files/softwares/spatialRLQ/spatialrlq.R
> 
> The function uses gpclib and only returns area or binary matrices (1 
> intersect, 0 not). It could be easily modify to return the intersection 
> of two polygons (remove the loops over the polylists and return interAB)
> 

Thanks, this will be useful, especially for users searching the archives! 
This also relates to the question of how much vector GIS functionality 
should be within R.

Roger

> 
> 
> Roger Bivand wrote:
> > On Tue, 20 Mar 2007, Sebastien wrote:
> >
> >   
> >> Dear helpers,
> >>
> >> I need to find the intersection of two polygons. I have two overlapping 
> >> polygons and I want to find the polygon corresponding to the common 
> >> part. Is there a fonction, like unionSpatialPolygons which can do that. 
> >> I have converted my polygons to "owin" and use the function 
> >> 'intersect.owin' in the pachage spatstat but it returns a binary matrice 
> >> of class "owin" or I would prefer a polygon.
> >>     
> >
> > No, there is not. I think that joinPolys() in the PBSmapping package can
> > do this, and you can get to the PBS PolySet representation using
> > SpatialPolygons2PolySet() in the maptools package, but then you'd need to
> > get back again to SpatialPolygons, which would involve coding. The 
> > alternative of using gpclib like unionSpatialPolygons() would also 
> > involve coding.
> >
> > Roger
> >
> >   
> >> Thanks for your help,
> >>
> >> Sébastien Ollier
> >>
> >> ####    ####    ####
> >> Sébastien Ollier
> >> Maître de Conférences
> >> Université Paris-Sud 11
> >> Faculté des Sciences d'Orsay
> >> Unité Ecologie, Systématique et Evolution
> >> Département Biodiversité, Systématique et Evolution
> >> UMR - 8079 UPS-CNRS-ENGREF
> >> Bât. 360, 91405 ORSAY CEDEX
> >> FRANCE
> >> Tel : 33 (0) 1 69 15 56 70
> >> Fax : 33 (0) 1 69 15 46 97
> >> http://www.ese.u-psud.fr/
> >> ####    ####    ####
> >>
> >>     
> >
> >   
> 
> 
> 

-- 
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