[R-sig-Geo] intersection of polygons

[Roger Bivand]

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.

Roger

>
> Thanks
> Ross
>
>
>
> 	[[alternative HTML version deleted]]
>
>

-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
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