Choropleth world map

Question

I have read so many threads and articles and I keep getting errors. I am trying to make a choropleth? map of the world using data I have from the global terrorism database. I wa

Answer 1

Something like this? This is a solution using rgdal and ggplot. I long ago gave up on using base R for this type of thing.

library(rgdal)        # for readOGR(...)
library(RColorBrewer) # for brewer.pal(...)
library(ggplot2)
setwd(" < directory with all files >")

gtd        <- read.csv("globalterrorismdb_1213dist.csv")
gtd.recent <- gtd[gtd$iyear>2009,]
gtd.recent <- aggregate(nkill~country_txt,gtd.recent,sum)
world      <- readOGR(dsn=".",
                      layer="world_country_admin_boundary_shapefile_with_fips_codes")

countries <- world@data
countries <- cbind(id=rownames(countries),countries)
countries <- merge(countries,gtd.recent, 
                   by.x="CNTRY_NAME", by.y="country_txt", all.x=T)
map.df <- fortify(world)
map.df <- merge(map.df,countries, by="id")
ggplot(map.df, aes(x=long,y=lat,group=group)) +
  geom_polygon(aes(fill=nkill))+
  geom_path(colour="grey50")+
  scale_fill_gradientn(name="Deaths",
                       colours=rev(brewer.pal(9,"Spectral")),
                       na.value="white")+
  coord_fixed()+labs(x="",y="")

There are several versions of the Global Terrorism Database. I used the full dataset available here, and then subsetted for year > 2009. So this map shows total deaths due to terrorism, by country, from 2010-01-01 to 2013-01-01 (the last data available from this source). The files are available as MS Excel download, which I converted to csv for import into R.

The world map is available as a shapefile from the GeoCommons website.

The tricky part of making choropleth maps is associating your data with the correct polygons (countries). This is generally a four step process:

1. Find a field in the shapefile attributes table that maps (no pun intended) to a corresponding field in your data. In this case, it appears that the field "CNTRY_NAME" in the shapefile maps to the field "country_txt" in gtd database.
2. Create an association between ploygon IDs (stored in the row names of the attribute table), and the CNTRY_NAME field.
3. Merge the result with your data using CNTRY_NAME and country_txt.
4. Merge the result of that with the data frame created using the fortify(map) - this associates ploygons with deaths (nkill).