How to calculate the area of polygon overlap in R?

Question

Does anyone know how to calculate the area in common between 2 or more polygons in R? I would like to have the output of such a calculation be the coordinates of a new polyg

Answer 1

EDIT: these days I would use the 'intersect', 'cover', 'erase', 'union' and related functions in the 'raster' package. They do the hard work to keep the top-level object and attributes.

ORIG: You could use the rgeos package with its gIntersection function. Successive calls between pairs and resulting intersections will get you there. See

library(rgeos)
?gIntersection

You will need to get into the structure of "SpatialPolygons" in the sp package to get the final coordinates. See the vignette("sp").

Answer 2

Just thought I would add the solution that I eventually used - the joinPolys function from the PBSmapping package.

Example:

library(PBSmapping)
p1 <- data.frame(PID=rep(1, 4), POS=1:4, X=c(1,1,6,6), Y=c(1,3,3,1))
p2 <- data.frame(PID=rep(2, 5), POS=1:5, X=c(4,4,8,8,6), Y=c(2,4,4,2,1))
p3 <- joinPolys(p1,p2)
x11()
par(mar=c(3,3,1,1))
plot(1,1,ylim=c(0,5),xlim=c(0,9), t="n", xlab="", ylab="")
polygon(p1$X, p1$Y, border=2)
polygon(p2$X, p2$Y)
polygon(p3$X, p3$Y, col=rgb(0,0,1,0.2))