U.S. DENSITY PATTERNS
Bill Rankin, 2008
On a density graph, population and land area are directly related by simple mathematics. But seeing both variables at the same time offers a few insights. First, there is something interesting happening at low densities. Below about 50 people per square mile, there are roughly equal areas of land at each density; between 50 and 250 people per square mile, there are roughly equal amount of population at each density. Neither of these patterns correspond to the expected density distribution around a central city; judging just on density alone, 250 people per square mile seems like the dividing line between city-centric and dispersed settlement. (Between 250 and 20,000 people per square mile, the empirical data correspond quite well to a simple model of density decreasing logarithmically away from a dense urban core.)
The bottom graph provides an easy way to see how much land is occupied by various proportions of the U.S. population. Half the population lives on 1% of the land, and the densest 10% of the population lives on just 0.035% of the land. This data drives home the fact that, whatever the problems with urban sprawl, impending land shortage is not one of them.