Understanding and Measuring Urban Expansion

The main thrust of our work has been a concentrated effort to measure different aspects of the spatial structure of a large number of urban areas the world over in a rigorous and consistent manner, in a way that can make it possible - for us as well as for others - to study them across time and space.

Our empirical study of urban expansion has benefited from previous efforts to measure urban sprawl while ignoring the negative connotations of the term. The academic literature includes numerous attempts to define and measure sprawl, and there is almost universal consensus on its key manifestations: endless cities, low densities, fuzzy boundaries between city and countryside, a polycentric urban structure, decentralized employment, single-use rather than mixed-use urban expanses, ribbons and commercial strips, scattered development, leapfrogging development, and the fragmentation of open space. Many of these attributes indeed characterize urban expansion everywhere and quite a few of them can be precisely measured so that we can compare them among cities or in a given city in two periods of time.

Following Galster et al. (2001), we define, map, and measure urban land cover and its attributes as patterns of urban land use - spatial configurations of a metropolitan area at a point in time - and urban expansion and its attributes as processes - changes in the spatial structure of cities over time. Patterns and processes are to be distinguished from the causes that bring about spatial patterns, or from the consequences of such patterns.

We have identified four discrete spatial attributes of cities that can now be mapped and measured systematically in all cities and countries. These attributes, studied both among cities and over time, provide a comprehensive characterization of urban expansion worldwide. They are: Urban land cover, density, fragmentation, and compactness.