Percentage of overlap between polygons

Question

I\'m new to R and it\'s my first time posting here, thought to let you guys know I\'m still a newbie in the R language with the hope it will simplify your answers. I\'m curr

Answer 1

For the fun of it, here's an example. Some folks here or on gis.stackexchange.com might have better ways in petto:

library(raster)
library(sp)
## example data:
p1 <- structure(c(0, 0, 0.4, 0.4, 0, 0.6, 0.6, 0), .Dim = c(4L, 2L), .Dimnames = list(NULL, c("x", "y")))
p2 <- structure(c(0.2, 0.2, 0.6, 0.6, 0, 0.4, 0.4, 0), .Dim = c(4L, 2L), .Dimnames = list(NULL, c("x", "y")))
p3 <- structure(c(0, 0, 0.8, 0.8, 0, 0.8, 0.8, 0), .Dim = c(4L, 2L), .Dimnames = list(NULL, c("x", "y")))
poly <- SpatialPolygons(list(Polygons(list(Polygon(p1)), "a"),Polygons(list(Polygon(p2)), "b"),Polygons(list(Polygon(p3)), "c")),1L:3L)
plot(poly)

## areas for the original shapes:
areas_poly <- vector(length = length(poly)) 
for (x in seq_along(poly)) areas_poly[x]<-area(poly[x])

## areas for the overlapping regions:
idx <- combn(seq_along(poly),2)
areas_intersect <- sapply(1:ncol(idx), function(x) {
  area(intersect(poly[idx[1,x]], poly[idx[2,x]])) 
})

## get overlaps in percentages:
overlap_perc <- 
  round(do.call(cbind, lapply(seq_len(ncol(idx)), function(x)  
  rbind(
    areas_intersect[x] / areas_poly[idx[1,x]] * 100, 
    areas_intersect[x] / areas_poly[idx[2,x]] * 100
  )
)), 2)

## into matrix form:
m <- matrix(100, ncol=length(poly), nrow=length(poly))
m[rbind(t(idx),t(idx)[,2:1])] <- as.vector(t(overlap_perc))
m
#       [,1]   [,2] [,3]
# [1,] 100.0  33.33  100
# [2,]  50.0 100.00  100
# [3,]  37.5  25.00  100