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 corr…

Accepted Answer

Here's a different approach. It works by:

- Converting the world map from the `maps` package into a `SpatialLines` object with a geographical (lat-long) CRS.

- Projecting the `SpatialLines` map into the Plate Carée (aka Equidistant Cylindrical) projection centered on the Prime Meridian. (This projection is very similar to a geographical mapping).

- Cutting in two segments that would otherwise be clipped by left and right edges of the map. (This is done using topological functions from the `rgeos` package.)

- Reprojecting to a Plate Carée projection centered on the desired meridian (`lon_0` in terminology taken from the `PROJ_4` program used by `spTransform()` in the `rgdal` package).

- Identifying (and removing) any remaining 'streaks'. I automated this by searching for lines that cross g.e. two of three widely separated meridians. (This also uses topological functions from the `rgeos` package.)

This is obviously a lot of work, but leaves one with maps that are minimally truncated, and can be easily reprojected using `spTransform()`. To overlay these on top of raster images with `base` or `lattice` graphics, I first reproject the rasters, also using `spTransform()`. If you need them, grid lines and labels can likewise be projected to match the `SpatialLines` map.

```
library(sp)
library(maps)
library(maptools)   ## map2SpatialLines(), pruneMap()
library(rgdal)      ## CRS(), spTransform()
library(rgeos)      ## readWKT(), gIntersects(), gBuffer(), gDifference()

## Convert a "maps" map to a "SpatialLines" map
makeSLmap <- function() {
    llCRS <- CRS("+proj=longlat +ellps=WGS84")
    wrld <- map("world", interior = FALSE, plot=FALSE,
            xlim = c(-179, 179), ylim = c(-89, 89))
    wrld_p <- pruneMap(wrld, xlim = c(-179, 179))
    map2SpatialLines(wrld_p, proj4string = llCRS)
}

## Clip SpatialLines neatly along the antipodal meridian
sliceAtAntipodes <- function(SLmap, lon_0) {
    ## Preliminaries
    long_180 <- (lon_0 %% 360) - 180
    llCRS  <- CRS("+proj=longlat +ellps=WGS84")  ## CRS of 'maps' objects
    eqcCRS <- CRS("+proj=eqc")
    ## Reproject the map into Equidistant Cylindrical/Plate Caree projection 
    SLmap <- spTransform(SLmap, eqcCRS)
    ## Make a narrow SpatialPolygon along the meridian opposite lon_0
    L  <- Lines(Line(cbind(long_180, c(-89, 89))), ID="cutter")
    SL <- SpatialLines(list(L), proj4string = llCRS)
    SP <- gBuffer(spTransform(SL, eqcCRS), 10, byid = TRUE)
    ## Use it to clip any SpatialLines segments that it crosses
    ii <- which(gIntersects(SLmap, SP, byid=TRUE))
    # Replace offending lines with split versions
    # (but skip when there are no intersections (as, e.g., when lon_0 = 0))
    if(length(ii)) { 
        SPii <- gDifference(SLmap[ii], SP, byid=TRUE)
        SLmap <- rbind(SLmap[-ii], SPii)  
    }
    return(SLmap)
}

## re-center, and clean up remaining streaks
recenterAndClean <- function(SLmap, lon_0) {
    llCRS <- CRS("+proj=longlat +ellps=WGS84")  ## map package's CRS
    newCRS <- CRS(paste("+proj=eqc +lon_0=", lon_0, sep=""))
    ## Recenter 
    SLmap <- spTransform(SLmap, newCRS)
    ## identify remaining 'scratch-lines' by searching for lines that
    ## cross 2 of 3 lines of longitude, spaced 120 degrees apart
    v1 <-spTransform(readWKT("LINESTRING(-62 -89, -62 89)", p4s=llCRS), newCRS)
    v2 <-spTransform(readWKT("LINESTRING(58 -89, 58 89)",   p4s=llCRS), newCRS)
    v3 <-spTransform(readWKT("LINESTRING(178 -89, 178 89)", p4s=llCRS), newCRS)
    ii <- which((gIntersects(v1, SLmap, byid=TRUE) +
                 gIntersects(v2, SLmap, byid=TRUE) +
                 gIntersects(v3, SLmap, byid=TRUE)) >= 2)
    SLmap[-ii]
}

## Put it all together:
Recenter <- function(lon_0 = -100, grid=FALSE, ...) {                        
    SLmap <- makeSLmap()
    SLmap2 <- sliceAtAntipodes(SLmap, lon_0)
    recenterAndClean(SLmap2, lon_0)
}

## Try it out
par(mfrow=c(2,2), mar=rep(1, 4))
plot(Recenter(-90), col="grey40"); box() ## Centered on 90w 
plot(Recenter(0),   col="grey40"); box() ## Centered on prime meridian
plot(Recenter(90),  col="grey40"); box() ## Centered on 90e
plot(Recenter(180), col="grey40"); box() ## Centered on International Date Line
```

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

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