What Micro-Mapping a City’s Density Reveals

By on August 13, 2019

From CityLab:  Density is one of the most important urban characteristics. A high-density city like New York looks, feels, and functions differently than a low-density city like Des Moines. Yet the textures of density within a single city can be just as varied as average densities between cities. Pockets of intense concentration or islands of relatively thin settlement sometimes bear little resemblance to the overall density of an urban area.

Since most people experience a city at the scale of a neighborhood or district, rather than the scale of metropolitan regions, paying attention to these fine-grained variations in urban form is more useful than broad-stroke statistics for understanding the everyday realities of population density. A new visualization I’ve put together helps picture these local patterns by examining density in American cities one square kilometer at a time. (Technically, these rectangles are 30 arc-second cells, which are roughly, but not exactly, one square kilometer.) This perspective lets us drill down to the micro-geography of density in American cities, with oftentimes surprising results, as in the densest grid cell of greater Salt Lake City (below), which isn’t high-rises but mobile home parks.

Most statistics that try to capture a single average measure, after all, tell us fairly little about spread or distribution. Consider economic indicators: If we know that the average income in a country is, say, $55,000, we don’t know anything about whether that country has a lot of middle-class earners or whether it’s polarized between extreme wealth and extreme poverty.

The same problem holds with density. Imagine one city where 50,000 people live in a 5-square-kilometer superblock of high-rises, surrounded by 20 square kilometers of parks, and another city where 50,000 people live in 25 square kilometers of low-density single-family homes. Both cities would have an average density of 2,000 people per square kilometer, but their actual morphologies could hardly be more different.

Density statistics can be even more misleading than averages like income or age, because the divisor in the equation is a geographic unit that’s inevitably drawn with debatable boundaries. For instance, the Los Angeles metropolitan statistical area (MSA) contains hundreds of square miles of mountains in the San Gabriel National Forest where nobody lives—so why shouldn’t the Boston MSA also include the hundreds of square miles of ocean in Boston Harbor where nobody lives? The Minneapolis–Saint Paul urbanized area includes several large lakes and even a National Wildlife Refuge, so why shouldn’t the Miami urbanized area include some chunk of the Everglades?

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