[R-sig-Geo] Rgshhs for getting maps around longitude 0

[Karl Ove Hufthammer]

Dear list members

The GSHHS map data uses longitudes from 0 to 360, which causes problems 
fetching maps around longitude 0. Example:

library(maptools)
gpclibPermit()
gshhs.c.b <- system.file("share/gshhs_c.b", package="maptools")
xl=c(-30,30)
yl=c(50,80)
rmap=Rgshhs(gshhs.c.b, xlim=xl, ylim=yl)
plot(rmap$SP)

There are no error messages, but some countries are just missing, e.g., 
Ireland, Iceland and Greenland. To get a map containing them, we have to use 
xlim=c(330,360). But what if we want to have both countries to the east and 
to the west of long = 0 on the same map (i.e., SpatialPolygons)?

Rgshhs takes a ‘shift’ argument, but this only changes the *output* 
coordinates from 0,360 to -180,180; it doesn’t change the accepted input 
variables.

Is it necessary to fetch the map twice? If so, which xlim values should be 
used, and how should the two maps be merged into one SpatialPolygons? Note 
that if I change ‘xlim’ to c(0,30), Great Britain is missing ... :-(

-- 
Karl Ove Hufthammer