[R-sig-Geo] intersection of polygons

[Edzer Pebesma]

On 03/29/2013 01:54 PM, Roger Bivand wrote:
> On Fri, 29 Mar 2013, Ross Ahmed wrote:
> 
>> Hi all,
>>
>> I have this list of polygons:
>>
>> library(sp)
>> p1 <- Polygon(cbind(c(-1.672487,-1.663689,-1.663185,-1.672258,-1.672487),
>>                    c(55.58951,55.58948,55.58249,55.58260,55.58951)))
>> p2 <- Polygon(cbind(c(-1.663689, -1.655508, -1.65514, -1.6633,
>> -1.663689),
>>                    c(55.589485, 55.589554, 55.582396, 55.582428,
>> 55.589485)))
>> p3 <- Polygon(cbind(c(-1.672224, -1.663046, -1.662525, -1.672137,
>> -1.672224),
>>                    c(55.582142, 55.582044, 55.575139, 55.575348,
>> 55.582142)))
>>
>> sp1 <- Polygons(list(Polygon(p1)),"p1")
>> sp2 <- Polygons(list(Polygon(p2)),"p2")
>> sp3 <- Polygons(list(Polygon(p3)),"p3")
>> spAll <- SpatialPolygons(list(sp1, sp2, sp3))
>> proj4string(spAll) <- CRS("+proj=longlat +ellps=WGS84 +datum=WGS84")
>>
>>
>> ?.and also this single polygon:
>>
>> poly1 <- Polygon(cbind(c(-1.666566, -1.659071, -1.658532, -1.666459,
>> -1.666566),
>>                       c(55.586296, 55.586357, 55.580414, 55.580505,
>> 55.586296)))
>> spoly1 <- Polygons(list(Polygon(poly1)),"poly1")
>> spoly1 <- SpatialPolygons(list(spoly1))
>> proj4string(spoly1) <- CRS("+proj=longlat +ellps=WGS84 +datum=WGS84")
>>
>>
>> I need to find the proportion of ?spoly1? that is overlapped by ?sp1?,
>> ?sp2?, ?sp3? and no polygon. The output should be something like:
>>
>> 0.40, 0.30, 0.20, 0.10
> 
> library(rgeos)
> gArea(spoly1)
> gArea(gIntersection(spAll, spoly1, byid=TRUE), byid=TRUE)/gArea(spoly1)
> 
> but note the warnings. You'll need to project both objects to get a
> proper measure.

Would errors instead of warnings not be more appropriate in this case?

-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de