Remote sensing study defines "edgy" cities

May 28, 2001

Sometimes you need to step back to see something clearly. Sometimes this means stepping way, way back. Otherwise, to update an old saying, you can't see the city for the houses.

In fact, in the relatively new science of studying urban areas that phenomenon is exactly the problem - when studied at the ground level, a city is such a complicated conglomeration of features that it is hard to make useful generalizations that allow researchers to define different kinds of cities - models that explain key differences that exist between one city and another.

What a difference 700 km. makes. Using data analysis techniques developed for research activities in the Central Arizona - Phoenix Long Term Ecological Research Project, Arizona State University geologists William Stefanov and Philip Christensen have turned to satellite data being gathered on 100 cities around the globe by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra spacecraft, a satellite in polar orbit. In a preliminary analysis of 12 cities, the researchers have found three significant configurations of urban development that they believe can be used to classify cities by their growth and density patterns.

The research represents the first results of the ASTER Urban Environmental Modeling Project and will be presented at a press briefing at the American Geophysical Union spring meeting in Boston on at 1 p.m. on May 29.

"We're going to collect data over each of 100 cities twice a year, both day and night," explains Stefanov, a researcher at in ASU's Department of Geological Sciences and the Center for Environmental Studies. "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 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."

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."

Using data from the first 12 cities the instrument has successfully scanned, 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. The second model is a place like Baltimore, 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 outwards gradually. Then the third model, represented by Riyadh and Madrid for example, where there seem to be characteristics of both."

Though the distinctions here 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. "Are there common features? How do cities change over time with respect to their environments? If there are not a lot of communalities, why are they different? Can you tie it down to one thing, like the geography - are cities decentralized because they have a lot of space? 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."

The cities that ASTER has studied so far include Albuquerque, NM; Bagdad, Iraq; Baltimore, MD; Chongqing, China; Istanbul, Turkey; Johannesburg, South Africa; Lisbon, Portugal; Madrid, Spain; Phoenix, AZ; Puebla, Mexico; Riyadh, Saudi Arabia; and Vancouver, British Columbia. A complete listing of all 100 cities to be studied can be found at

ASTER obtains high-resolution (15 to 90 square meters per pixel) images of the Earth in 14 different wavelengths of the electromagnetic spectrum, ranging from visible reflectance to thermal infrared emmission. Scientists use ASTER data to create detailed maps of land surface temperature, emissivity, reflectance, and elevation. ASTER is the only high spatial resolution instrument on the Terra platform.

"ASTER is Landsat on steroids," said Stefanov. "It's a wonderful tool for studying surface materials." The ASTER instrument was built in Japan for the Ministry of International Trade and Industry. A joint United States/Japan Science Team is responsible for instrument design, calibration, and data validation.

The instrument is one of five major packages on the Terra Spacecraft, the flagship of NASA's Earth Observing System, launched in December 1999 with an expected six-year mission.
Source: William Stefanov, 480-965-5507

Arizona State University

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