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.)

This exercise is part of the course

Visualizing Geospatial Data in R

Exercise instructions

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.

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

library(sp)
library(tmap)

# Switch to a Hobo–Dyer projection
tm_shape(countries_spdf) +
  tm_grid(n.x = 11, n.y = 11) +
  tm_fill(col = "population", style = "quantile")  +
  tm_borders(col = "burlywood4") 

# Switch to a Robinson projection
tm_shape(countries_spdf) +
  tm_grid(n.x = 11, n.y = 11) +
  tm_fill(col = "population", style = "quantile")  +
  tm_borders(col = "burlywood4") 

# Add tm_style("classic") to your plot

Edit and Run Code

This exercise is part of the course

Visualizing Geospatial Data in R

IntermediateSkill Level

4.6+

Start Course for Free

We'll dive in by displaying some spatial data -- property sales in a small US town -- using ggplot2 and we'll introduce you to the ggmap package as a quick way to add spatial context to your plots. We'll talk about what makes spatial data special and introduce you to the common types of spatial data we'll be working with throughout the course.

Exercise 1: Introduction to spatial data Exercise 2: Grabbing a background map Exercise 3: Putting it all together Exercise 4: Insight through aesthetics Exercise 5: Useful get_map() and ggmap() options Exercise 6: Different maps Exercise 7: Leveraging ggplot2's strengths Exercise 8: A quick alternative Exercise 9: Common types of spatial data Exercise 10: Drawing polygons Exercise 11: Choropleth map Exercise 12: Raster data as a heatmap

You can get a long way with spatial data stored in data frames, but it makes life easier if they are stored in special spatial objects. In this chapter we'll introduce you to the spatial object classes provided by the sp package, particularly for point and polygon data. You'll learn how to explore and subset these objects by exploring a world map. The reward for learning about these object classes: we'll show you the package tmap which requires spatial objects as input, but makes creating maps really easy! You'll finish up by making a map of the world's population.

Exercise 1: Introducing sp objects Exercise 2: Let's take a look at a spatial object Exercise 3: What's inside a spatial object?Exercise 4: A more complicated spatial object Exercise 5: sp and S4 Exercise 6: Walking the hierarchy Exercise 7: Further down the rabbit hole Exercise 8: More sp classes and methods Exercise 9: Subsetting by index Exercise 10: Accessing data in sp objects Exercise 11: Subsetting based on data attributes Exercise 12: tmap, a package that works with sp objects Exercise 13: Introduction to tmap Exercise 14: Building a plot in layers Exercise 15: Why is Greenland so big?

Current Exercise

Exercise 16: Saving a tmap plot

While the sp package provides some classes for raster data, the raster package provides more useful classes. You'll be introduced to these classes and their advantages and then learn to display them. The examples continue with the theme of population from Chapter 2, but you'll look at some much finer detail datasets, both spatially and demographically. In the second half of the chapter you'll learn about color -- an essential part of any visual display, but especially important for maps.

Exercise 1: The raster package Exercise 2: What's a raster object?Exercise 3: Some useful methods Exercise 4: A more complicated object Exercise 5: A package that uses Raster objects Exercise 6: Color scales Exercise 7: Pick the right palette Exercise 8: Adding a custom continuous color palette to ggplot2 plots Exercise 9: Custom palette in tmap Exercise 10: More about color scales Exercise 11: An interval scale example Exercise 12: A diverging scale example Exercise 13: A qualitative example

In this chapter you'll follow the creation of a visualization from raw spatial data files to adding a credit to a map. Along the way, you'll learn how to read spatial data into R, more about projections and coordinate reference systems, how to add additional data to a spatial object, and some tips for polishing your maps.

Exercise 1: Reading in spatial data Exercise 2: Reading in a shapefile Exercise 3: Reading in a raster file Exercise 4: Getting data using a package Exercise 5: Coordinate reference systems Exercise 6: Merging data from different CRS/projections Exercise 7: Converting from one CRS/projection to another Exercise 8: Adding data to spatial objects Exercise 9: The wrong way Exercise 10: Checking data will match Exercise 11: Merging data attributes Exercise 12: A first plot Exercise 13: Polishing a map Exercise 14: Subsetting the neighborhoods Exercise 15: Adding neighborhood labels Exercise 16: Tidying up the legend and some final tweaks Exercise 17: Wrap up