[R] Faster union of polygons?

Remko Duursma remkoduursma at gmail.com
Wed Jun 2 03:53:39 CEST 2010

```Dear R-helpers,

thanks for yesterday's speeding-up tip. Here is my next query:

I have lots of polygons (not necessarily convex ones, and they never
have holes) given by x,y coordinates.

I want to get the polygon that is the union of these polygons. This is
my current method, but I am hoping there is a faster method (up to
thousands of polygons, each with ca. 40 xy points).

Example:

library(gpclib)

# A polygon
leaf <- structure(c(0, 1, 12.9, 16.5, 18.8, 17, 16.8, 15.5, 12.1, 8.2,
6.3, 5, 2, 0, -1.5, -4.3, -6.6, -10.3, -14.8, -19.4, -22.2, -23.5,
-22.2, -17.6, -7.8, 0, 0, -2.4, 2.8, 8.9, 19.9, 33.9, 34.8, 40.4,
49.7, 69.2, 77.4, 83.4, 91.4, 99, 92.8, 87.3, 81.2, 71.1, 57.6,
45.4, 39.2, 26, 15.6, 5.3, 0.6, 0), .Dim = c(26L, 2L), .Dimnames = list(
NULL, c("X", "Y")))

# Lots of polygons:
releaf <- function(leaf)cbind(leaf[,1]+rnorm(1,0,50),leaf[,2]+rnorm(1,0,50))
leaves <- replicate(500, releaf(leaf), simplify=FALSE)

# Make into gpc.poly class:
leaves <- lapply(leaves, as, "gpc.poly")

# Make union .....
system.time({
leavesoutline <- union(leaves[[1]], leaves[[2]])
for(i in 3:length(leaves))leavesoutline <- union(leavesoutline, leaves[[i]])
})

# Check it:
plot(leavesoutline)

thanks!

Remko

-------------------------------------------------
Remko Duursma
Research Lecturer

Centre for Plants and the Environment
University of Western Sydney
Hawkesbury Campus
Richmond NSW 2753

Dept of Biological Science
Macquarie University
North Ryde NSW 2109
Australia

Mobile: +61 (0)422 096908
www.remkoduursma.com

```