[R-sig-Geo] intersection of polygons

[Ross Ahmed]

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
>> 
>> 
>> 
>>    [[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
>