Plot a world map with a specified grid with R

Question

I am trying to use wrld_simpl from maptools package in order to plot a piece of world map with a longitude/latitude grid.

For example, I have a netCDF file with longitude and latitude, I create a matrix with all the points I can have on the grid:

lat <- get.var.ncdf(nc,"lat")
long <- get.var.ncdf(nc,"lon")
pts <- SpatialPoints(expand.grid(long,lat), CRS(proj4string(wrld_simpl)))

Now I want to plot the countries whose coordinates are on my grid.

I don't know how to do with wrld_simpl attributes!

Answer 1

Did you look at maps mapproject packages ? You can draw any coast line very quickly with the desired resolution and it comes with projections and grid. You can apply projections to one of your object ( expand.grid(long,lat) ). Example in Antartica:

library("mapproj")
m <- map( "world", "Antarctica", plot=FALSE)
map("world", "Antarctica", proj="conic", param=-90, fill=TRUE, col="gray")
map.grid(m, nx=4, ny=6, col="black")

front.lim <- data.frame(x=seq(-180, 180, length.out=1000),
                        y=rep(-62, 1000))
front.lim <- mapproject(front.lim$x, front.lim$y, projection="conic", parameters=-90)
lines(front.lim$x, front.lim$y, col="red", lwd=2)

If you have already plotted the ncdf matrix, you can simply add map(add=TRUE) . It will read the x and y limits and draw the borders in this area.

plot(c(-10, 45), c(30, 60), type="n", xlab="", ylab="") # image(ncdf)
map(add=TRUE, fill=TRUE, col="gray")
abline(v=seq(-10, 45, 7.5), lty=2)
abline(h=seq(-30, 60, 7.5), lty=2)