R Beginners - Plotting Locations on to a World Map

This post is targeted at those who are just getting started plotting on maps using R.

The relevant libraries are: maps, ggplot2, ggmap, and maptools. Make sure you install them.

The Problem

Let's take a fairly simple use case: We have a few points on the globe (say Cities) that we want to mark on the map.

The ideal and natural choice for this would be David Kahle's ggmap package. Except that there is a catch. ggmap doesn't handle extreme latitudes very well. If you are really keen on using ggmap, you can do it by following the technique outlined in this StackOverflow response.
 If ggmap is not mandatory, there are simpler ways to do the same.

First, let's set up our problem. We'll take 5 cities and plot them on a world map.

	library("ggmap")
	library(maptools)
	library(maps)
	visited <- c("SFO", "Chennai", "London", "Melbourne", "Johannesbury, SA")
	ll.visited <- geocode(visited)
	visit.x <- ll.visited$lon
	visit.y <- ll.visited$lat
	#> dput(visit.x)
	#c(-122.389979, 80.249583, -0.1198244, 144.96328, 28.06084)
	#> dput(visit.y)
	#c(37.615223, 13.060422, 51.5112139, -37.814107, -26.1319199)

view raw worldmap_plotting.R hosted with ❤ by GitHub

Method 1: Using the maps Package

	#USING MAPS
	map("world", fill=TRUE, col="white", bg="lightblue", ylim=c(-60, 90), mar=c(0,0,0,0))
	points(visit.x,visit.y, col="red", pch=16)

view raw gistfile1.r hosted with ❤ by GitHub

This results in:

Which might be enough. However, if you take the few extra steps to plot using ggplot, you will have much greater control for what you want to do subsequently.

Method 2: Plotting on a World Map using ggplot 

 

	#Using GGPLOT, plot the Base World Map
	mp <- NULL
	mapWorld <- borders("world", colour="gray50", fill="gray50") # create a layer of borders
	mp <- ggplot() + mapWorld
	#Now Layer the cities on top
	mp <- mp+ geom_point(aes(x=visit.x, y=visit.y) ,color="blue", size=3)
	mp

view raw gistfile1.r hosted with ❤ by GitHub

This results in: