intersection of polygons

Many thanks Roger. 

I thought proj4string(spAll) <- CRS("+proj=longlat +ellps=WGS84 +datum=WGS84") took care of projection, but obviously not. 

How do I project?

Ross

On 29 Mar 2013, at 12:54, Roger Bivand <Roger.Bivand at nhh.no> 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.

Roger

Thanks
Ross



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-- 
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