Image: NASA

When it comes to studying large urban centers, getting the proverbial big picture can be difficult. So researchers from Arizona State University took a step back700 kilometers back, in fact. To better understand the composition of cities around the world, they analyzed images taken by NASA's Terra spacecraft, which uses the Advanced Spaceborne Thermal Emissions and Reflection Radiometer(ASTER) to gather detailed information about Earth's surface.

"We're going to collect data over each of 100 cities twice a year, both day and night," Arizona State geologist William Stefanov explains. "The whole idea is to be able to classify the land cover of those cities, differentiating vegetative versus nonvegetative, urban versus nonurban, developed versus undeveloped areas, and to track them over six years to begin to see how these cities are changing over time and how they're interacting with their surrounding environment."

Based on surface patterns, the researchers defined boundaries between different kinds of land use. These boundaries in turn led them to create three categories for the 12 cities they recently analyzed. "Decentralized cities, such as Phoenix and Albuquerque, lack well-defined urban centers or cores," Stefanov says. "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. The second model is a place such as Baltimore (top), which is a centralized city. 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 outward gradually. The third model, represented by Riyadh and Madrid (bottom), for example, [has] characteristics of both."

Although some of this may seem intuitive, the researchers point out that their results represent the first specific data of this kind. "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 environments," Stefanov says. "There could be any number of social reasons why cities are structured the way they are. What this does is give a real, physical, measurable basis to those theories.