How do you combine a map with complex display of points in ggplot2?

Question

I\'m trying to plot points from study sites with a background map of Africa. I can create the two independently, but I am having a hard time overlaying them on top of eacho

Answer 1

Try something like this. Seems to work for me. I think some of your lat-long coordinates are wrong though. The fill colour for geom_point is currently set to Tot437 so you might want to change that.

library(ggplot2)
library(rgdal)

africa <- readOGR("c:/test", layer = "Africa")
africa.map = fortify(africa, region="COUNTRY")

africa.points = read.table("c:/test/SPM-437-22Nov12.txt", header = TRUE, sep = ",")
names(africa.points)[which(names(africa.points) == 'Longitude')] <- 'long' # rename lat and long for consistency with shp file
names(africa.points)[which(names(africa.points) == 'Latitude')] <- 'lat'

ggplot(africa.map, aes(x = long, y = lat, group = group)) +
    geom_polygon(colour = "black", size = 1, fill = "white", aes(group = group)) +
    geom_point(data = africa.points, aes(x = long, y = lat, fill = Tot437, group = NULL), size = 4, shape = 21, colour = "black", size = 3)

Incidentally, looking at your map you may have difficulty getting a good detailed view of individual areas, so one way to tackle that would be by subsetting, in this case with the data frames. You could do this:

africa.map <- africa.map[africa.map$id == 'Madagascar', ]
africa.points <- africa.points[africa.points$Country == 'Madagascar', ]
ggplot(africa.map, aes(x = long, y = lat, group = group)) +
    geom_polygon(colour = "black", size = 1, fill = "white", aes(group = group)) +
    geom_point(data = africa.points, aes(x = long, y = lat, fill = Tot437, group = NULL), size = 2, shape = 21, colour = "black", size = 2)

...which should get you something similar to this: