Easier Composite U.S. Choropleths with albersusa

March 29, 2016

(This article was first published on R – rud.is, and kindly contributed to R-bloggers)

Folks who’ve been tracking this blog on R-bloggers probably remember this post where I showed how to create a composite U.S. map with an Albers projection (which is commonly referred to as AlbersUSA these days thanks to D3).

I’m not sure why I didn’t think of this earlier, but you don’t need to do those geographical machinations every time you want a prettier & more inclusive map (Alaska & Hawaii have been states for a while, so perhaps we should make more of an effort to include them in both data sets and maps). After doing the map transformations, the composite shape can be saved out to a shapefile, preferably GeoJSON since (a) you can use geojsonio::geojson_write() to save it and (b) it’s a single file vs a ZIP/directory.

I did just that and saved both state and country maps out with FIPS codes and other useful data slot bits and created a small data package : albersusa : with some helper functions. It’s not in CRAN yet so you need to devtools::install_github("hrbrmstr/albersusa") to use it. The github repo has some basic examples, heres a slightly more complex one.

Mapping Obesity

I grabbed an obesity data set from the CDC and put together a compact example for how to make a composite U.S. county choropleth to show obesity rates per county (for 2012, which is the most recent data). I read in the Excel file, pull out the county FIPS code and 2012 obesity rate, then build the choropleth. It’s not a whole lot of code, but that’s one main reason for the package!

library(ggplot2)   # devtools::install_github("hadley/ggplot2") only if you want subtitles/captions
library(albersusa) # devtools::install_github("hrbrmstr/albersusa")
# get the data and be nice to the server and keep a copy of the data for offline use
URL <- "http://www.cdc.gov/diabetes/atlas/countydata/OBPREV/OB_PREV_ALL_STATES.xlsx"
fil <- basename(URL)
if (!file.exists(fil)) download.file(URL, fil)
# it's not a horrible Excel file, but we do need to hunt for the data
# and clean it up a bit. we just need FIPS & 2012 percent info
wrkbk <- read_excel(fil)
obesity_2012 <- setNames(wrkbk[-1, c(2, 61)], c("fips", "pct"))
obesity_2012$pct <- as.numeric(obesity_2012$pct) / 100
# I may make a version of this that returns a fortified data.frame but
# for now, we just need to read the built-in saved shapefile and turn it
# into something ggplot2 can handle
cmap <- fortify(counties_composite(), region="fips")
# and this is all it takes to make the map below
gg <- ggplot()
gg <- gg + geom_map(data=cmap, map=cmap,
                    aes(x=long, y=lat, map_id=id),
                    color="#2b2b2b", size=0.05, fill=NA)
gg <- gg + geom_map(data=obesity_2012, map=cmap,
                    aes(fill=pct, map_id=fips),
                    color="#2b2b2b", size=0.05)
gg <- gg + scale_fill_viridis(name="Obesity", labels=percent)
gg <- gg + coord_proj(us_laea_proj)
gg <- gg + labs(title="U.S. Obesity Rate by County (2012)",
                subtitle="Content source: Centers for Disease Control and Prevention",
           caption="Data from http://www.cdc.gov/diabetes/atlas/countydata/County_ListofIndicators.html")
gg <- gg + theme_map(base_family="Arial Narrow")
gg <- gg + theme(legend.position=c(0.8, 0.25))
gg <- gg + theme(plot.title=element_text(face="bold", size=14, margin=margin(b=6)))
gg <- gg + theme(plot.subtitle=element_text(size=10, margin=margin(b=-14)))



Note that some cartographers think of this particular map view the way I look at a pie chart, but it’s a compact & convenient way to keep the states/counties together and will make it easier to include Alaska & Hawaii in your cartographic visualizations.

The composite GeoJSON files are in:

  • system.file("extdata/composite_us_states.geojson", package="albersusa")
  • system.file("extdata/composite_us_counties.geojson", package="albersusa")

if you want to use them in another program/context.

Drop an issue on github if you want any more default fields in the data slot and if you “need” territories (I’d rather have a PR for the latter tho :-).

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