on steroids," said Arizona State University geologist William Stefanov. "It's a wonderful tool for studying surface materials."When looked at from ground level, a city looks so complicated that it is hard to discern between different kinds of urban environments and create models of how and why they evolve. Such models could explain key differences that exist between one city and another and help determine the cause of those differences.
So Stefanov and colleague Philip Christensen developed a method of using ASTER data to classify cities by their growth and density patterns. They plan to monitor 100 world cities over the next six years to see how they change.
ASTER records 14 different measurements that show the differences between a forested area and a paved city block, from how much light is reflected to the amount of heat emitted or reflected.
"The whole idea is to be able to classify the land cover of those cities, differentiating vegetative vs. non-vegetative, urban vs. non-urban, developed vs. undeveloped areas," Stefanov said.
The researchers presented their first results, preliminary data on 12 cities, at a media briefing at the American Geophysical Union spring meeting Tuesday in Boston.
Profiling a city's texture
Looking at differences in the "texture" created by picture elements produced by light and dark surface materials, the researchers have found a method that allows them to define boundaries between different kinds of land cover in urban areas the "edges" of a city.
"By running some simple algorithms over the image you can pull out the edgy' parts," said Stefanov. "What you can do once you have this data is to do image transects across it and look at it in profile. Once you see it in profile, you begin to see patterns in the density of the city itself."
The researchers defined three major classes: decentralized cities, centralized cities and intermediate cities, which combine certain centralized and decentralized features.
"Decentralized cities, like Phoenix and Albuquerque, lack well-defined urban centers or cores. The city is pretty much built up all the way out, and then you come to a well-defined boundary where it becomes a natural area," Stefanov said.
"The second model is a place like Baltimore, which is a centralized city," he said. "When you look at the texture of that city you see that it has a very well-defined urban center and the edginess or density of structures grades outwards gradually. Then the third model, represented by Riyadh and Madrid for example, where there seem to be characteristics of both."
Data to support the obvious
Though the distinctions may seem obvious, Stefanov points out that until now cities have only been described by general characterizations, rather than by specific data. The ASTER data provides a specific benchmark for modeling cities and a scientific basis for further study.
"An urban ecologist would want to know why these cities have these clear differences," Stefanov said. "The whole idea is that by getting an idea of how our cities are actually structured we can start to see commonalities in how humans like to engineer their environment."
The research might help answer questions about whether a city sprawls out mostly due to the surrounding geography or for more social or cultural reasons.
ASTER was built in Japan for the Ministry of International Trade and Industry. The instrument is one of five major packages on the Terra spacecraft,
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