Use different center than the prime meridian in plotting a world map

Question

I am overlaying a world map from the maps package onto a ggplot2 raster geometry. However, this raster is not centered on the prime meridian (0 deg), but on 180 deg (roughly the Bering Sea and the Pacific). The following code gets the map and recenters the map on 180 degree:

require(maps) world_map = data.frame(map(plot=FALSE)[c("x","y")]) names(world_map) = c("lon","lat") world_map = within(world_map, {   lon = ifelse(lon < 0, lon + 360, lon) }) ggplot(aes(x = lon, y = lat), data = world_map) + geom_path() 

which yields the following output:

Quite obviously there are the lines draw between polygons that are on one end or the other of the prime meridian. My current solution is to replace points close to the prime meridian by NA, replacing the within call above by:

world_map = within(world_map, {   lon = ifelse(lon < 0, lon + 360, lon)   lon = ifelse((lon < 1) | (lon > 359), NA, lon) }) ggplot(aes(x = lon, y = lat), data = world_map) + geom_path() 

Which leads to the correct image. I now have a number of question:

1. There must be a better way of centering the map on another meridian. I tried using the orientation parameter in map, but setting this to orientation = c(0,180,0) did not yield the correct result, in fact it did not change anything to the result object (all.equal yielded TRUE).
2. Getting rid of the horizontal stripes should be possible without deleting some of the polygons. It might be that solving point 1. also solves this point.

Answer 1

This may be somewhat tricky but you can do by:

mp1 <- fortify(map(fill=TRUE, plot=FALSE)) mp2 <- mp1 mp2$long <- mp2$long + 360 mp2$group <- mp2$group + max(mp2$group) + 1 mp <- rbind(mp1, mp2) ggplot(aes(x = long, y = lat, group = group), data = mp) +    geom_path() +    scale_x_continuous(limits = c(0, 360)) 

By this setup you can easily set the center (i.e., limits):

ggplot(aes(x = long, y = lat, group = group), data = mp) +    geom_path() +    scale_x_continuous(limits = c(-100, 260)) 

UPDATED

Here I put some explanations:

The whole data looks like:

ggplot(aes(x = long, y = lat, group = group), data = mp) + geom_path() 

but by scale_x_continuous(limits = c(0, 360)), you can crop a subset of the region from 0 to 360 longitude.

And in geom_path, the data of same group are connected. So if mp2$group <- mp2$group + max(mp2$group) + 1 is absent, it looks like: