LIKE a good Impressionist painting, a city often requires a bit of distance before its viewer can get a feel for the whole picture. Terra, the newest earth-science spacecraft launched by NASA, America's space agency, can provide 700km-worth of distance; and William Stefanov, a geologist at Arizona State University, is using this to develop a better understanding of how people create their cities. At this week's meeting of the American Geophysical Union, in Boston, Dr Stefanov explained how he used data from a satellite-borne instrument called ASTER (Advanced Spaceborne Thermal Emission and Reflection radiometer) to study 12 cities and to classify their growth patterns.
Urban planners have long known that cities have different characteristics, but the view from space makes assessing those characteristics faster and more accurate. Dr Stefanov and his colleagues (or, rather, their computers) looked for the boundaries between densely developed areas, which reflect a lot of light because of all the edges and flat surfaces that people like to create, and those with more vegetation, which appear dark by comparison, as nature prefers curves.
By focusing on the edges separating dark from light areas, the researchers classified their dozen cities into three types. Six of them, Baghdad, Baltimore, Istanbul, Lisbon, Puebla and Vancouver, are “centralised”: most of their development is concentrated around a “downtown” area, and there is a gradual fading away from this with distance. Three, Albuquerque, Johannesburg and Phoenix, were “non-centralised”: developed more or less evenly over the entire area, but with an abrupt edge. The other three, Chongqing, Madrid and Riyadh, were a mixture of the two types.
Other methods of measuring a city's composition and growth are less precise and more time-consuming than Dr Stefanov's. They usually involve a combination of aerial photography, ground measurements and door-to-door canvassing. Because that takes so long, it cannot be done frequently. The data from ASTER provide a relatively simple and much faster alternative.
Ultimately, the team plans to study 100 cities over six years, to try to work out why different cities grow in the way they do. The hope is to tease out the relative contributions to the process of geography, political history, climate, social structure and economics. Working with local experts in each city should provide some answers—and a better idea of how each Impressionistic dot makes up a part of the whole.
This article appeared in the Science & technology section of the print edition under the headline "Getting off the ground"