The map above shows the routes flown by the top 7 airlines (by international passenger distance flown). The base map shows large urban areas and I have attempted to make it look a bit like the beautiful “Earth at Night” composite image produced by NASA. You can clearly see a relationship between where people live and where the big carriers fly to across Europe and the US but India and much of China have relatively few routes. I expect much of the slack is picked up by smaller airlines in these countries but they must represent key growth areas the world economy becomes increasingly driven by the east. This map isn’t meant to be comprehensive- I just wanted to make another example of a visualisation with ggplot2.
How I did it
Plotting great circles has become an increasingly popular thing to do with R (because they look cool) and the excellent flight path data freely available from the OpenFlights website provides a neat data source to play around with. There are several tutorials out there but none of them (to my knowledge) apply apply colour value to the arcs based on a relevant variable in the datafile or make much of the underlying base map. For the Tufte fans out there this is means an important opportunity for maximising data ink has been missed.
The first step was to calculate the great circles with the flight data. The Anthrospace blog has a good tutorial on this so I won’t replicate the code here. I would warn you that it is a little tricky to sort out. For the R pros out there if you have a refinement on the code please comment below.
The next step was to source the world urban area boundaries. These can be found on the Natural Earth Data website. Direct link. In the code below I have simplified these and coloured them in the plot to reflect the colours in the NASA image. I have also coloured the background and continents accordingly. Without the great circle arcs your basemap should look a bit like this (I’m really pleased with how it came out).
With this layer in place you can now draw whatever you want on top of it. In this case it is the flight arcs. I then added a few annotations and moved the key etc in Inkscape for the final image.
Load in your great circles (see above for link on how to do this). You need a file that has long, lat, airline and group. The group variable is produced as part of the Anthrospace tutorial.
Get a world map
Load in your urban areas shapefile from Natural Earth Data
Simplify these using the gsimplify function from the rgeos package
Fortify them for use with ggplot2
This step removes the axes labels etc when called in the plot.
xquiet yquiet<-scale_y_continuous("", breaks=NA, lim=c(70,-70))
Create a base plot
Then build up the layers
wrld<-c(geom_polygon(aes(long,lat,group=group), size = 0.1, colour= "#090D2A", fill="#090D2A", alpha=1, data=worldmap))
Bring it all together with the great circles
base+wrld+urb+geom_path(data=gcircles, aes(long,lat, group=group, colour=airline),alpha=0.1, lineend="round",lwd=0.1)+coord_equal()+quiet+opts(panel.background = theme_rect(fill='#00001C',colour='#00001C'))