Mapping Urban Sprawl

An unprecedented survey of urban sprawl in the continental United States is challenging conventional notions about development. The research, published in the May issue of the Quarterly Journal of Economics, offers a detailed view of how land use varies across the country and, for the first time, explains what may be causing the variation.

'There was a lot of talk about urban sprawl and there were no facts. We put this data set together so that we could get some facts,' said Matthew Turner, associate professor of economics at the University of Toronto.

Questions and opinions about urban sprawl and its causes have been around for years. But according to Turner, the opinions and policies arising from these debates are based largely on speculation.

To produce meaningful data, the researchers merged high-altitude photography from around 1976 with satellite images from 1992 -- the most recent images available. They divided the space into 8.7 billion grid cells, each representing 900 square meters (a little less than a quarter of an acre).

The scientists pinpointed development by measuring the percentage of impermeable cover, such as that created by buildings and concrete, producing a sprawl index. The higher the index, the more scattered a city. For example, among metropolitan areas with a population over one million, Atlanta has an index of 56 percent, while New York has an index of 20 percent.


via Discovery Channel