Measuring and Modeling Global Urban Expansion1 by

The process of urban expansion – conversion of land to urban uses – has been viewed with sufficient alarm to prompt attempts by the public sector to regulate it for over 300 years, and probably much longer. Historically, this expansion was concentrated in the most developed and most affluent countries of the world, where increasing urbanization was associated with economic development. In the past several decades, urban expansion has been perceived as a significant problem globally, as the share of population in urban areas increases and urbanization increases or even accelerates in countries around the world. Urban areas may then expand at astonishing rates in cities that often lack either preparation or planning for the expansion. At the same time, for reasons unrelated to the pace of urbanization, global capacity to monitor the process of urban expansion through remote sensing has expanded dramatically. It is now possible to have access to roughly 20 years of satellite images of virtually any city, with data collected in a consistent and comparable fashion (unlike aerial photographs, which are available for an even longer time span in selected areas). This paper reports some results from a study based on a global sample of 120 cities, chosen to be globally representative of the urban population in places having population of at least 100,000. Satellite image