The Aura of Urbanization   Page 2Page 4

Since researchers first suspected that this trend was taking place, the single biggest problem in tracking it has been in finding a way to measure the full extent of urbanization across very large regions, such as whole continents. Several years ago, Imhoff came across a solution. He discovered satellite images displaying the illumination cities and towns generate at night. The images were taken by the Defense Meteorological Satellite Program’s Operational Linescan System (OLS). The satellite network was originally designed to aid in aircraft navigation by detecting the lunar illumination off of nighttime clouds. What the Air Force realized is that on evenings when there was a new moon, the satellite was sensitive enough to record the illumination from city lights. Over a period of several new moons, the data the satellite retrieved could be pieced together to produce a global image of city lights.

  North American Lights

Using computer algorithms, Imhoff figured out a way to create maps of the approximate population density across an entire country or continent from the images (see Bright Lights, Big City). "We essentially scaled back on the brightness levels of the imaging data," says Imhoff. The first full map of population density he constructed was of the United States. With help from U.S. Census Bureau statistics, the Goddard team was able to classify all land area in the United States into three categories—urban, peri-urban, and non-urban areas.

An urban region, Imhoff elaborates, is defined as an area with 1000 people or more per square mile. These are regions where humans have developed and completely transformed the natural ecosystem. Any scientist looking at a region classified as urban on the map can be fairly certain that there are parking lots, office buildings, some strip malls, and maybe a fast-food restaurant or two. Peri-urban areas, on the other hand, have only been lightly populated. They usually consist of farmland, light suburban development or small towns and are classified as having an average of 100 people per square mile. In most instances, this is the type of land development that occurs as cities expand. Finally, non-urban areas are regions such as central Montana and western Maine, where only ten people or less live per square mile.

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Based upon satellite measurements of city lights, this image is a map of the urban population density of North America. Red yellow and green are urban areas, and blue is peri-urban. The city light data is laid over elevation data (black is sea level, light grey is over 10,000 feet). Most major cities are in level areas along an ocean bay, large lake, or navigable river. (Image courtesy Marc Imhoff, NASA GSFC, and Flashback Imaging Corporation, Ontario, Canada)

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