Frequently asked questions

1. Can you make a map with such-and-such a color scheme? I'm kind of busy at the moment, so I'm probably not going to be making a lot more maps. However, you can easily make maps with other color schemes yourself. It's easy to take the purple maps, separate out the blue and red channels, and then remap them any way you like. This is just a couple of clicks in Photoshop. (In fact, I used the excellent free Photoshop clone Gimp for my image manipulation, which does the job very well. You can download it from here.)

2. Can you make a map showing such-and-such data? Many people have asked for maps showing number of people who voted, number of registered voters, differences between the candidates' votes, and all sorts of other things. Once again, I'm probably not going to be doing this soon, but I certainly encourage others to make such maps. My software for creating cartograms is freely available for download here.

3. Do you have maps for previous elections? Yes. There are maps for the 2004 and 2006 elections. None before that – sorry.

4. Where are Alaska and Hawaii? Not on the maps. I know. Sorry. There are some technical problems with non-contiguous cartograms that make it difficult to produce nice maps. But I'm working on it. Stay tuned.

5. The shapes of the cartograms look different from those for the previous election. What's up with that? It's true the shapes are a bit different even though the population data used are the same. The reason is that I've made some improvements to the computer algorithm in the last couple of years. The old algorithm tended to create a "bloated" look when coastal regions like Florida had large populations – they would expand like balloons and become bulbous. In the new version of the algorithm such regions retain their overall shape better and there is less bloating. Compare Florida in the 2004 and 2008 maps to see what I mean.

6. Can I use your figures in my magazine, newspaper, mailing list, web page, artwork, wall hanging, interpretive dance, etc? Absolutely. The maps and the accompanying text are released under a Creative Commons License that allows for their free distribution and use in derivative works. I would appreciate hearing from you if you wish to make use of them, but it is not required under the terms of the license.

7. How exactly do you make these maps? So you want the technical stuff, huh? OK, well, let's see. The cartograms were made using the diffusion method of Gastner and Newman. The population data and geographic boundaries were taken from the most recent (year 2000) US Census.

   The calculation of the cartograms involves allowing the population to diffuse in the two-dimensional space of the map, carrying the boundaries of the states or counties with it, until it reaches a uniform equilibrium. The diffusion equation is integrated in Fourier space, where it takes a particularly simple form: the initial density function is evaluated on a 4608x3072 lattice, transformed using a two-dimensional fast Fourier transform, convolved with a Gaussian kernel and then back-transformed to give the diffusion field at an arbitrary later time. I used closed (Neumann) boundary conditions at the edges of the map, meaning that the Fourier transform in this case is a discrete cosine transform.

   The diffusion field is then used to calculate the diffusion velocity as a function of position and the velocity integrated over time to give the displacement of the map features. The integration is performed using a fourth-order Runge-Kutta integrator with an adaptive step size and local extrapolation. The entire calculation took about ten minutes for each map on a standard desktop computer running the Fedora Linux operating system. The basic images were created using a specially written rendering program and some artistic refinements were added using Gimp, a free image manipulation program.

8. Have these maps been getting press coverage? The maps I and two colleagues made for the 2004 election have received wide attention, and were used as part of the 2008 election night coverage on ABC and on the BBC in Britain. They have also appeared in the Washington Post, on CNN Headline News, in The Guardian, and on Salon.com among other places. You are welcome to use any of the maps in your publication as well; see above for the licensing details.