[R-sig-Geo] recentering a map to arbitrary center while keeping polygons / nowrapRecenter()

[Peter Ellis (Wellington)]

Hi

I'm trying to recenter a world map without breaking the polygons.  If x1 is a SpatialPolygons or SpatialPolygonsDataFrame object that I have (for example) created from a shapefile then:

	plot(nowrapRecenter(x1))

does exactly what I want, with one exception that by making the boundaries 0 and 360 degrees longitude we end up with the UK on the far right of the map and the rest of Europe on the far left (the "world2" map in the maps package has the same issue).  So what I really want is something that can frame the map left to right with an arbitrary set of horizontal coordinates - say -20 to 340.  

>From reading the help file I would have thought that

	plot(nowrapRecenter(x1, offset=-20))

would do the job, but this does two things that surprise me:  
1) The objects on the map don't shift to the right, relative to when offset=0
2) more scratched polygons appear - apparently from polygons that go across the longitude=0

Reproducible example:

library(maps)
library(maptools)
library(rgeos)
library(rgdal)
library(gpclib)

world <- map("world", fill=TRUE, col="transparent", plot=FALSE)
worldSpP <- map2SpatialPolygons(world, world$names, CRS("+proj=longlat"))

# remove incomplete polygons that go over the offset
worldSpP <- worldSpP[-grep("Greenland", row.names(worldSpP)),]
worldSpP <- worldSpP[-grep("Iceland", row.names(worldSpP)),]
worldSpP <- worldSpP[-grep("Antarctica", row.names(worldSpP)),]
worldSpPnr <- nowrapRecenter(worldSpP, offset=-20)
plot(worldSpPnr, col=colors())

My very basic guess of what might be going wrong is that nowrapRecenter has basically done what it intends to, but the plot method is still trying to draw the map from 0 to 360.  However, fixing this goes beyond my troubleshooting powers.


Alternatively, there's two different answers on StackOverflow that will redraw a world map with an arbitrary center very neatly (one using ggplot2 and one using sp), but in both cases the country boundary information is basically lost:
http://stackoverflow.com/questions/10620862/use-different-center-than-the-prime-meridian-in-plotting-a-world-map

So my question reduces to how do I use nowrapRecenter() to center a map arbitrarily; or is there another way to move a world map like that, while keeping country information.

Thanks!

PE



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