rgisr-spr-maptoolscartogram

Population-weighted polygon distortion (cartograms)


I'm trying to create a map in R that conveys both the shape of underlying geometry (i.e., the physical boundaries) and the relative importance of the object in terms of an associated value.

For concreteness, I'd like to focus on reproducing (a version of) the following map* (the shapes, not so much the colors since I can't find the polling data):

A distorted map of the United States titled "2012 Electoral College Map", subtitled "The size of each state is distorted to emphasize its share of electoral votes". The states are colored red or blue according to whether that state voted for Mitt Romney or Barack Obama in 2012, respectively. The legend notes this, as well as the total electoral vote count (277 for Obama, 261 for Romney), and a note that 270 electoral votes are required to win.

I also don't want to bother with getting Alaska and Hawaii to appear below the US instead of in their geodesically correct locations.

I'm only as far as getting the data merged with the weights, done for example as follows:

1. Get polygons

library(maptools)
library(data.table) #not strictly necessary but I prefer it
#US states downloaded (500k resolution) from:
#https://www.census.gov/geo/maps-data/data/cbf/cbf_state.html
us.states<-
  readShapePoly("~/Desktop/cb_2014_us_state_5m.shp")

setDT(us.states@data)

#for getting rid of territories, AK, HI
states<-sprintf("%02d",1:59)
ak.hi<-c("02","15")

us.states.contig<-
  us.states[us.states@data$STATEFP %in% 
              setdiff(states,ak.hi),]

#Unadorned plot
plot(us.states.contig)
text(coordinates(us.states.contig),
     us.states.contig@data[,paste0(STUSPS)],
     cex=.7)

A plain, undistorted map of the continental United States. White polygons, black borders, annotated with two-letters state postal codes.

2. Add Electoral College data

#scraped from government page
library(rvest) #only necessary to scrape table
electoral.college.url<-
  paste0("http://www.archives.gov/federal-register/",
         "electoral-college/allocation.html")

electoral.college.dt<-
  (html(electoral.college.url) %>%
     html_nodes("table"))[[5]] %>% 
  html_table()
setDT(electoral.college.dt)
setnames(electoral.college.dt,c("State","Votes"))

#merge into geodata
us.states.contig@data<-
  copy(us.states.contig@data)[
    electoral.college.dt,electoral.votes:=i.Votes,
    on=c(NAME="State")]

#plot, coloring each state by size
states.ranked<-
  us.states.contig@data[,rank(electoral.votes,
                              ties.method="first")]
cols<-colorRampPalette(c("red","blue"))(51)[states.ranked]

plot(us.states.contig,col=cols)

The same undistorted map of states, but this time, each state is colored red, blue, or purple. Red means "few electoral college votes" (for example, North and South Dakota are starkly red); blue means "many electoral college votes" (for example, Texas and California are starkly blue); colors on the gradient in between are "medium electoral college votes"

This is all well and good--with a glance at this map, we can tell which states have high & low representation in the electoral college. But what if (as in our target map) we wanted to represent another variable with the color of the state?

3. Add 2012 Elections outcomes

#scrape again
#2012 Election Results by State
election.wiki<-
  paste0("https://en.wikipedia.org/wiki/",
         "United_States_presidential_election,_2012")
         
results<-
  html(election.wiki) %>%
  html_node(xpath='//*[@id="mw-content-text"]/div[22]/table') %>%
  html_table()
#eliminate second header row, delete final row,
#  keep only the important columns
results.trim<-results[2:(nrow(results)-1),c(1,4,21)]
colnames(results.trim)<-c("name","pct","abbr")
results.dt<-setDT(results.trim)
#data idiosyncrasies, see Wiki page
results.dt<-results.dt[!grepl("–",abbr)|grepl("a",abbr)]
results.dt[grepl("–",abbr),abbr:=gsub("–.*","",abbr)]
results.dt[,"pct":=as.numeric(gsub("%","",pct))]

#merge
us.states.contig@data<-
  copy(us.states.contig@data
       )[results.dt,vote.pct:=i.pct,
         on=c(STUSPS="abbr")]
                              
pcts<-us.states.contig@data[,vote.pct]
cols<-c("red","blue")[(pcts>=50)+1L]
tx.col<-c("white","black")[(cols=="red")+1L]
plot(us.states.contig,col=cols)
text(coordinates(us.states.contig),
     us.states.contig@data[,paste0(STUSPS)],
     col=tx.col)

The undistorted states again, this time exactly red or blue according to the electoral outcome.

This last graph gets at the crux of the issue. The first graph presented is far superior in the sense that we can sense from the percentage red vs. blue of the map whether the Republicans or Democrats won; this last map is misleading because the most Republican states are also the most sparsely-populated.

Is there any way to create a distorted version of this map which conveys the relative importance of each state in the electoral college? I couldn't find any help online, perhaps mainly because I don't know if there's a standard name for this type of graph.

*This map was found here; I've seen similar size-distorted maps before, e.g. in The Economist. It appears it's based on the work of Dr. Sam Wang at Princeton's Election Consortium and was produced by Drew Thaler.


Solution

  • Following the advice of package maintainer @chkaiser, I've sought out and finally discovered a way to do this within R. This blog post was a tremendous help and the getcartr package is fantastic.

    First, get the Rcartogram and getcartr packages from GitHub:

    remotes::install_github("omegahat/Rcartogram")
    remotes::install_github('chrisbrunsdon/getcartr', subdir='getcartr')
    library(Rcartogram)
    library(getcartr)
    

    Now simply plug & chug:

    us.states.contig.carto = quick.carto(
      us.states.contig,
      us.states.contig@data$electoral.votes
    )
    plot(us.states.contig.carto, col = cols)
    text(
      coordinates(us.states.contig.carto),
      us.states.contig@data[ , paste0(STUSPS)],
      col = tx.col
    )
    

    And just like that we have our cartogram:

    cartogram