Why is Greenland so big?

Take a closer look at the plot. Why does Greenland look bigger than the contiguous US when it's actually only about one-third the size?

When you plot longitude and latitude locations on the x- and y-axes of a plot, you are treating 1 degree of longitude as the same size no matter where you are. However, because the earth is roughly spherical, the distance described by 1 degree of longitude depends on your latitude, varying from 111km at the equator, to 0 km at the poles.

The way you have taken positions on a sphere and drawn them in a two dimensional plane is described by a projection. The default you've used here (also known as an Equirectangular projection) distorts the width of areas near the poles. Every projection involves some kind of distortion (since a sphere isn't a plane!), but different projections try to preserve different properties (e.g. areas, angles or distances).

In tmap, tm_shape() takes an argument projection that allows you to swap projections for the plot.

(Note: changing the projection of a ggplot2 plot is done using the coord_map() function. See ?coord_map() for more details.)

To help you see the differences between projections, we've added a tm_grid() layer which adds equispaced longitude and latitude lines to the plot.

Within your tm_shape() call:

Try a Hobo–Dyer projection (projection = "hd"), designed to preserve area.
In a second plot, try a Robinson projection (projection = "robin"), designed as a compromise between preserving local angles and area.
Just for fun, repeat the previous plot, but add tm_style("classic") to see how tmap can control all aspects of the maps display.

Exercise

Why is Greenland so big?

Instructions