You’ve likely seen nighttime satellite pictures of the earth before—images that show Tokyo and New York blazing away and a swath of darkness over the Sahara.
These images offer a vivid picture of human activity around the world, and now it turns out they may be instrumental in solving a very practical problem in the social sciences: how to define where metropolitan boundaries begin and end.
Currently social scientists lack a consistent way of doing that, which makes comparisons across studies and between metro regions difficult. But a paper published earlier this year in The Professional Geographer suggests that light measurements from nighttime satellite images can be used to define metropolitan boundaries consistently in cities around the world.
As you’d expect, light intensity is highest in city centers and diminishes as you move outward. The authors, led by the prolific Richard Florida of the University of Toronto, had to make a judgment call about the light threshold that marks the end of a metro region. But the beauty of their method is that once the light threshold is set, it gives you a consistent measure of metropolitan size. (More consistent, for example, than metrics like commuting time, which don’t translate well from city to city.)
Having gathered light data, the authors then use it to estimate metro-area economic activity: They multiply national GDP by the share of light emissions coming from a given metro-area, and arrive at a measure they call “light-based regional product.” From this, the authors conclude that the 10 largest metropolitan areas in the world—which contain only 2.6 percent of the world’s population—account for 21.2 percent of global economic activity.
Half the fun of the article is poring over its rankings of metro-areas. Tokyo-Kawasaki-Yokohama is the brightest—and richest—in the world, with an economic output of $1.9 trillion, nearly twice the output of the New York-Philadelphia-Newark corridor, which comes in at number two.
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