Creating Effective Cartograms20 Dec 2008
[caption id="attachment_375" align="alignright" width="210" caption="A Standard 2008 Election Map (Upper), and a Cartogram skewed by Population (Lower)"] [/caption]
Cartograms, or visualizations of an area skewed by some variable, are a powerful tool to control for disparities over a large area, especially with respect to politics.
A relatively large but sparsely populated area will dominate a standard projection, whereas a cartogram allows populated areas to be warped to show their true influence (See sidebar).
Applications of Cartograms:
Socio-economic data is the most obvious use case for cartograms, particularly data from the United Nations and National Elections.
<!-- more -->The cartogram strategy can be applied to just about any area visualization, and works particularly well where there are major disparities between area size and overall effect.
Algorithms and Tools:
A general outline of the cartogram creation process, as well as some excellent alternatives to the standard 'skewed border' approach can be found at "Scaling Counties in a Checkerboard State" over at style.org.
Actual implementation of cartograms seems to often follow the algorithm first published here. Desktop versions in Java (with code) are available here and here. There's a rundown of more methods at indiemaps.
[caption id="attachment_386" align="alignright" width="210" caption="A Rescaled Election Cartogram. Can You Identify the State?"][/caption]
Another post at style.org looks at alternate skewing methods, namely squarifying the areas in question, and then expanding them by a fixed ratio with respect to the data.
Some of the most beautiful work in cartograms comes from the SASI group and Dr. Daniel Dorling.
Indiemaps blog has a number of posts on cartograms, including the promise of python source code, and some great visuals.
I think cartograms are an untapped resource for commerical data, especially for geographically disbursed figures like sales or conversion rates. Hopefully in the future we'll see more tools centered around this resource, as well as a decent online generator.