Question

how do I plot an area around a set of points on a map in R? e.g.

map('world')
map.axes()
p <- matrix(c(50, 50, 80, 100, 70, 40, 25, 60), ncol=2) # make some points
points(p, pch=19, col="red")

polygon(p, col="blue")

… which gives me a polygon with a vertex at each of the points, but it looks rather crappy. Is there any way to “smooth” the polygon into some sort of curve?

Answer 1

One option is to make a polygon bounded by a Bézier curve, using the bezier function in the Hmisc package. However I cannot get the start/end point to join up neatly. For example:

## make some points
p <- matrix(c(50, 50, 80, 100, 70, 40, 25, 60), ncol=2)
## add the starting point to the end
p2 <- cbind(1:5,p[c(1:4,1),])
## linear interpolation between these points                            
t.coarse <- seq(1,5,0.05)
x.coarse <- approx(p2[,1],p2[,2],xout=t.coarse)$y
y.coarse <- approx(p2[,1],p2[,3],xout=t.coarse)$y
## create a Bezier curve                                           
library(Hmisc)
bz <- bezier(x.coarse,y.coarse)
library(maps)
map('world')
map.axes()
polygon(bz$x,bz$y, col=rgb(0,0,1,0.5),border=NA)