Friday, November 9, 2012

When choropleth maps deceive

See the map at Huffington Post

On election night, I experienced the misleading power of choropleth maps that many fine cartography textbooks (such as this one and this one) warn about. I was sitting in the living room with my wife. She was watching TV and I had four browser windows open in my laptop, displaying Twitter, CNN, The New York Times, and the Huffington Post map above. Suddenly, she stared at the computer screen and said: "I don't get how it is possible that Obama is winning the election; according to that map, more people are voting for Romney. I see more red than blue in there." That's actually the impression the map caused when Florida was still undecided.

I then reminded her of how the Electoral College works —she is not American, like me— and showed her one of my favorite election graphics ever (below), which combines different kinds of maps to better display the 2008 results. I wonder if cartograms, like the one of the bottom of it, should not be mandatory for this kind of coverage, just to take population into account.

Graphic by The New York Times