Chapter 8 Mapping with style
“How beautiful the world was when one looked at it without searching, just looked, simply and innocently.”
— Hermann Hesse, Siddartha
“You can’t judge a book by it’s cover but you can sure sell a bunch of books if you have a good one.”
— Jayce O’Neal
Now that we have learned the basics of creating a beautiful map in
ggplot2 it is time to look at some of the more particular things we will need to make our maps extra stylish. There are also a few more things we need to learn how to do before our maps can be truly publication quality.
If we have not yet loaded the
tidyverse let’s do so.
# Load libraries library(tidyverse) library(scales) library(ggsn) # Load Africa map load("data/africa_map.RData")
8.1 Default maps
In order to access the default maps included with the
tidyverse we will use the function
ggplot() + borders() + # The global shape file coord_equal() # Equal sizing for lon/lat
Jikes! It’s as simple as that to load a map of the whole planet. Usually we are not going to want to make a map of the entire planet, so let’s see how to focus on just the area around South Africa.
sa_1 <- ggplot() + borders(fill = "grey70", colour = "black") + coord_equal(xlim = c(12, 36), ylim = c(-38, -22), expand = 0) # Force lon/lat extent sa_1
That is a very tidy looking map of South(ern) Africa without needing to load any files.
8.2 Specific labels
A map is almost always going to need some labels and other visual cues. We saw in the previous section how to add site labels. The following code chunk shows how this differs if we want to add just one label at a time. This can be useful if each label needs to be different from all other labels for whatever reason. We may also see that the text labels we are creating have
\n in them. When R sees these two characters together like this it reads this as an instruction to return down a line. Let’s run the code to make sure we see what this means.
sa_2 <- sa_1 + annotate("text", label = "Atlantic\nOcean", x = 15.1, y = -32.0, size = 5.0, angle = 30, colour = "navy") + annotate("text", label = "Indian\nOcean", x = 33.2, y = -34.2, size = 5.0, angle = 330, colour = "springgreen") sa_2
8.3 Scale bars
With our fancy labels added, let’s insert a scale bar next. There is no default scale bar function in the
tidyverse, which is why we have loaded the
ggsn package. This package is devoted to adding scale bars and North arrows to
ggplot2 figures. There are heaps of options so we’ll just focus on one of them for now. It is a bit finicky so to get it looking exactly how we want it requires some guessing and checking. Please feel free to play around with the coordinates below. We may see the list of available North arrow shapes by running
sa_3 <- sa_2 + scalebar(x.min = 22, x.max = 26, y.min = -36, y.max = -35, # Set location of bar dist = 200, height = 1, st.dist = 0.8, st.size = 4, # Set particulars dd2km = TRUE, model = "WGS84") + # Set appearance north(x.min = 22.5, x.max = 25.5, y.min = -33, y.max = -31, # Set location of symbol scale = 1.2, symbol = 16) sa_3
In order to inset a smaller map inside of a larger map we must first create the smaller map. We have already loaded just such a map of Africa so we will use that for this example.
And now to inset this map of Africa into our map of southern Africa we will need to learn how to create a ‘grob’. This is very simple and does not require any extra work on our part. Remember that
ggplot2 objects are different from normal objects (i.e. dataframes), and that they have their own way of storing and accessing data. In order to convert any sort of thing into a format that ggplot understands we convert it into a grob, as shown below. Once converted, we may then plop it onto our figure/map wherever we please. Both of these steps are accomplished with the single function
annotation_custom(). This is also a good way to add logos or any other sort of image to a map/figure. You can really go completely bananas. It’s even possible to add GIFs. Such happy. Much excite. Very wonderment.
sa_4 <- sa_3 + annotation_custom(grob = ggplotGrob(africa_map), xmin = 20.9, xmax = 26.9, ymin = -30, ymax = -24) sa_4
8.5 Rounding it out
There are a lot of exciting things going on in our figure now. To round out our adventures in mapping let’s tweak the lon/lat labels to a more prestigious convention. There are two ways to do this. One of which requires us to install the
scales package. Don’t worry, it’s a small one!
sa_final <- sa_4 + scale_x_continuous(breaks = seq(16, 32, 4), labels = scales::unit_format("°E", sep = ""), position = "bottom") + scale_y_continuous(breaks = seq(-36, -24, 4), labels = c("36.0°S", "32.0°S", "28.0°S", "24.0°S"), position = "right") + labs(x = "", y = "") sa_final
And lastly we save the fruits of our labours.
ggsave(plot = sa_final, filename = "figures/southern_africa_final.pdf", height = 6, width = 8)