R: plotting neighbouring countries using maptools

Question

Say I am plotting countries on a world map using maptools, if I were to plot a country, is there a way of plotting the countries that border this country in a different colour? I am using the shapefile wrld_simpl that comes with maptools, so say I plot China:

plot(wrld_simpl[wrld_simpl$NAME=='China',], col='red', add=T)

is there a way I can get it to plot all the bordering countries to China. I want to be able to do this for lots of different countries so I'd ideally want a general solution, not one specific to just China.

Answer 1

How about gTouches or gIntersects in rgeos?

library(rgeos)
library(maptools)
wc <- subset(wrld_simpl, NAME == "China")
world <- subset(wrld_simpl, !NAME == "China")

Create a vector to store the test:

tst <- logical(nrow(world))

Do the test:

for (i in 1:nrow(world)) {
    tst[i] <- gTouches(wc, world[i,])
}

See the result:

levels(world$NAME)[world$NAME[tst]]
[1] "India"  "Russia"

plot(wc)
plot(world[tst, ], add = TRUE, col = "grey")

(No further correspondence on world affairs will be entered into, but gIntersects seems to give a better answer).

I strongly advise you to treat these built-in data sets with caution, you certainly need to get your hands on reliable data if you are to use such a tool for any non-trivial purpose. Geometry and data are tricky enough already. :)

for (i in 1:nrow(world)) {
     tst[i] <- gIntersects(wc, world[i,])
 }
length(levels(world$NAME)[world$NAME[tst]])
[1] 14
 plot(world[tst, ], col = "firebrick")
 plot(wc, add = TRUE, col = "grey")