Making inset maps for publication

DATE: 2019-02-20

AUTHOR: John L. Godlee

In scientific publications, often there is a map to describe the study location. For example in Burton et al. (2013), which is the first publication I found with an inset map by searching for the key word "inset map".

Example inset map

I wanted to be able to make these maps directly in R, rather than manually exporting each map image and overlaying them in PowerPoint or Photoshop. The maps I make are in {ggplot2}, so the examples here all use ggplot2. The example involves making a map of Angola and adding some points of key cities, then adding an inset map showing the location of Angola in the world.

First, load some packages:

library(ggplot2)
library(rworldmap)

Then create fortified polygons by grabbing country data from {rworldmap}, using the getMap() function:

world <- getMap(resolution = "low")
world_f <- fortify(world)

angola <- world[world@data$ADMIN == "Angola", ]
angola_f <- fortify(angola)

Then, make the inset world map, by wrapping the normal ggplot() call in ggplotGrob():

world_grob <- ggplotGrob(
    ggplot() +
    geom_polygon(data = world_f, aes(x = long, y = lat, group = group), fill = NA, colour = "black") +
    geom_polygon(data = angola_f, aes(x = long, y = lat), fill = "#F2C81F") +
    theme_void() +
    coord_quickmap() +
    theme(panel.background = element_rect(fill = "white", colour = "black"))
)

This saves the ggplot object in a format that can be added later as an annotation_custom().

Then, create some city location data:

city_location <- data.frame(city = c("Luanda", "Lubango"),
    long = c(13.267, 13.500),
    lat = c(-8.845, -14.918))

Then create a ggplot() object of the country of Angola, with the city locations added as geom_point():

angola_ggplot <- ggplot() +
    geom_polygon(data = angola_f, aes(x = long, y = lat),
    	fill = "#F2C81F", colour = "black") +
    geom_point(data = city_location, aes(x = long, y = lat), colour = "black", size = 5) +
    geom_label(data = city_location, aes(x = long, y = lat, label = city), hjust = -0.2) +
    theme_classic() +
    coord_quickmap() +
    labs(x = "Longitude", y = "Latitude")

Finally, plot angola_ggplot again, using annotation_custom() to add the world_grob:

angola_ggplot +
    annotation_custom(grob = world_grob,
    	xmin = 18, xmax = 26,
    	ymin = -9, ymax = -4)

annotation_custom() allows you to set the x and y location of the grob, using the coordinate system on the base plot, which in this case is decimal latitude and longitude.

My inset map in R

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