Appendix 4
Methodology for Urban Extent Maps* Objectives City-level data for African cities are scarce and spatial information even more so. City leaders and city and other decision makers need data to make informed planning and investment decisions. As part of the economic and sector work on integrated urban water management (IUWM), the World Bank undertook a major data collection exercise. As part of this we developed (based on satellite imagery) a set of maps of urban extent for 31 cities in Africa. The maps show historic urban extent from 1990 (when available) to today and illustrate likely future urban extent by 2025. The purpose hereof is to contribute to better decision making by providing an illustrative tool of the likely spatial consequences of continued urban growth.
Methodology Spatial modeling of urban growth has been developing since the 1960s, and has increasingly moved toward the use of cellular automata models. These models represent space as a grid where each cell in that grid is subject to a certain possibility of transition in use based on a defined set of spatial rules and probabilities. These simple systems can give rise to * For more details, see Duncan, Blankespoor and Engstrom. “Urban Extent Map for 31 cities in Africa”. World Bank, Washington, DC. Available at http://water.worldbank.org/AfricaIUWM.
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