Most maps are curious combinations of the natural and the man-made, charts that show us the rivers, lakes, and mountains that have developed across millenia as well as the lines we humans have established, in much more recent history, to divide them all up. But this map by Brandon Martin-Anderson, a graduate student at MIT’s Changing Places lab, shows one thing and one thing only: people, as counted in the most recent U.S. and Canadian censuses.





Martin-Anderson’s map (which is really worth a look in its full, zoomable glory) is dizzyingly dense, with some three hundred million data points, but it’s also exceedingly straightforward. One dot per person–nothing else. The designer says he got the idea when he was looking at a series of race and ethnicity dot density maps created by designer Eric Fischer. Curious about what his own neighborhood would look like in greater detail, he started plotting census data. “I started with the University District neighborhood in Seattle,” he says, “but then I was curious about Seattle. Then I was curious about western Washington, then Washington, then the whole West Coast, then the U.S.”

At first glance, the picture it shows is understandable enough. Major cities are dense pockets of black, with more uninhabited white space cropping up as you move from east to west. But it’s remarkable just how pronounced that drop-off is moving from states like Minnesota, Iowa, and Missouri to the Dakotas, Nebraska, and Kansas, and the states beyond. As Martin-Anderson points out, that abrupt drop-off lines up neatly with the average precipitation experienced by those areas. “I love this a lot,” he says, “because it illustrates the extent to which humans in large numbers act like something so simple and biological–like a field of grass growing under the reach of a sprinkler.”

Other observations from the mapmaker? For one thing, the map shows just how sparse northern Canada really is; 64% of the country’s population actually resides south of Seattle. It also illustrates some unique regional trends. The band of black along the Eastern Seaboard isn’t much of a surprise, but the metropolitan axis running from Atlanta to Raleigh-Durham is surprisingly dense.

For Martin-Anderson, the process of making the map was also enlightening. Sifting through the census data, he found that the highest density blocks were prisons, dorms, barracks, homeless shelters, and luxury apartments. “It’s an extremely heterogeneous collection of outliers,” he says. “People are prone to making politically charged statements about the goodness or badness of population density, but it’s very difficult to make any true and wide-reaching statements about areas with extremely high population density.”





But the project raises other questions still, mainly about the types of maps we make and use as a society. If the concept behind the dot-a-person map is so straightforward, and the results so insightful, why don’t we see them more often?

The answer, says Martin-Anderson, can be traced to the fact that we’ve only recently become familiar with an easy-to-use tool for making sense of insanely dense, multi-scale maps: pinch-to-zoom.