Question

I am trying to create a thematic map showing all 50 US states, but I am having trouble relocating Alaska and Hawaii in a reliable way. I have a couple of ideas but none of them work well. I will demonstrate them now.

First we need to import the data; using the data in the maps package is not enough because it does not include Hawaii and Alaska.

setwd(tempdir())
download.file("https://dl.dropbox.com/s/wl0z5rpygtowqbf/states_21basic.zip?dl=1", 
              "usmapdata.zip", 
              method = "curl")
# This is a mirror of http://www.arcgis.com/home/item.html?
# id=f7f805eb65eb4ab787a0a3e1116ca7e5
unzip("usmapdata.zip")

require(rgdal)
all_states <- readOGR("states_21basic/", "states")

require(ggplot2); require(maptools); require(rgeos); require(mapproj);
all_states <- fortify(all_states, region = "STATE_NAME")

Now we define some plot aesthetics:

p <- ggplot() + geom_polygon( 
  aes(x=long, y=lat, group = group, fill = as.numeric(as.factor(id))), 
  colour="white", size = 0.25
) + coord_map(projection="azequalarea") + 
scale_fill_gradient(limits = c(1,50))

Now we remove all background etc so they don’t clash when we overlap the non-contiguous states:

p <-   p + theme(axis.line=element_blank(),
            axis.text.x=element_blank(),
            axis.text.y=element_blank(),
            axis.ticks=element_blank(),
            axis.title.x=element_blank(),
            axis.title.y=element_blank(),
            panel.background=element_blank(),
            panel.border=element_blank(),
            panel.grid.major=element_blank(),
            panel.grid.minor=element_blank(),
            plot.background=element_blank())

Using viewports

The first idea I had was to use viewports:

AK <- p %+% subset(all_states, id == "Alaska") + theme(legend.position = "none")
HI <- p %+% subset(all_states, id == "Hawaii") + theme(legend.position = "none")
contiguous <- p %+% subset(all_states, id != "Alaska" & id != "Hawaii")

grid.newpage()
vp <- viewport(width = 1, height = 1)
print(contiguous, vp = vp)
subvp1 <- viewport(width = 0.25, height = 0.25, x = 0.18, y = 0.33)
print(AK, vp = subvp1)
subvp2 <- viewport(width = 0.12, height = 0.12, x = 0.32, y = 0.27)
print(HI, vp = subvp2)

This looks nice but it is not satisfactory because it is very sensitive to slight changes in the figure, for example resizing or changes in the size and shape of the legend.

Manually moving Alaska and Hawaii

all_states_AKHImoved <- within(all_states, {
  lat[id == "Alaska"] <- lat[id == "Alaska"] - 45
  long[id == "Alaska"] <- long[id == "Alaska"] + 40
  lat[id == "Hawaii"] <- lat[id == "Hawaii"] + 0
  long[id == "Hawaii"] <- long[id == "Hawaii"] + 70
})
p %+% all_states_AKHImoved

This is not satisfactory because Alaska is usually not to scale on most US maps so it looks very big. Also relocating Alaska and Hawaii changes the distortion introduced by the map projection.

Question

Does anyone have any better approaches?

Answer 1

Here’s how to do it by projecting and transforming. You will need:

require(maptools)
require(rgdal)

fixup <- function(usa,alaskaFix,hawaiiFix){

  alaska=usa[usa$STATE_NAME=="Alaska",]
  alaska = fix1(alaska,alaskaFix)
  proj4string(alaska) <- proj4string(usa)

  hawaii = usa[usa$STATE_NAME=="Hawaii",]
  hawaii = fix1(hawaii,hawaiiFix)
  proj4string(hawaii) <- proj4string(usa)

  usa = usa[! usa$STATE_NAME %in% c("Alaska","Hawaii"),]
  usa = rbind(usa,alaska,hawaii)

  return(usa)

}

fix1 <- function(object,params){
  r=params[1];scale=params[2];shift=params[3:4]
  object = elide(object,rotate=r)
  size = max(apply(bbox(object),1,diff))/scale
  object = elide(object,scale=size)
  object = elide(object,shift=shift)
  object
}

Then read in your shapefile. Use rgdal:

us = readOGR(dsn = "states_21basic",layer="states")

Now transform to equal-area, and run the fixup function:

usAEA = spTransform(us,CRS("+init=epsg:2163"))
usfix = fixup(usAEA,c(-35,1.5,-2800000,-2600000),c(-35,1,6800000,-1600000))
plot(usfix)

The parameters are rotations, scaling, x and y shift for Alaska and Hawaii respectively, and were obtained by trial and error. Tweak them carefully. Even changing Hawaii’s scale parameter to 0.99999 sent it off the planet because of the large numbers involved.

If you want to turn this back to lat-long:

usfixLL = spTransform(usfix,CRS("+init=epsg:4326"))
plot(usfixLL)

But I’m not sure if you need to use the transformations in ggplot since we’ve done that with spTransform.

You can now jump through the ggplot2 fortify business. I’m not sure if it matters for you but note that the order of the states is different in the usfix version – Alaska and Hawaii are now the last two states.