If you did not find the geospatial data you need in existing R packages (see the map section), you need to find this information elsewhere on the web.
Usually, you will find it as a shape file format. This format is composed by several files that you need to keep together in the same folder.
Note: if you found a
.geoJSON file, read
You need to dig the internet to find the shape file you are interested in. For instance, this URL will redirect you to a zipped shape file containing the worl boundaries.
You can download it and unzip it with R:
# Download the shapefile. (note that I store it in a folder called DATA. You have to change that if needed.) download.file("http://thematicmapping.org/downloads/TM_WORLD_BORDERS_SIMPL-0.3.zip" , destfile="DATA/world_shape_file.zip") # You now have it in your current working directory, have a look! # Unzip this file. You can do it with R (as below), or clicking on the object you downloaded. system("unzip DATA/world_shape_file.zip") # -- > You now have 4 files. One of these files is a .shp file! (TM_WORLD_BORDERS_SIMPL-0.3.shp)
rgdal package offers the
readOGR() function that allows to read shapefile using the
As a result you get a geospatial object (
my_spdf here) that
contains all the information we need for further mapping. Please try th
following command to understand how this object works:
summary(my_spdf): tells you the max and min coordinates, the kind of projection in use
length(my_spdf): how many regions you have
head(my_spdf@data): the firs few rows of the data slot associated with the regions
plot() function knows how to plot a
geospatial object. Thus you just need to pass it
my_spdf and add a couple of options to customize the
It is totally possible (and advised imo) to build the map with ggplot2. However, ggplot2 takes as input data frames, not geospatial data.
my_spdf thus needs to be transformed using the
tidy() function of the
broom package. The
region argument of this function expect one of the
column name if the @data slot. It will
be the region name in the new dataframe.
Once the data frame is created, it is plotted using the
geom_polygon() function as described below.