168 The Future of Water in African Cities
• The final outputs of this stage were vector polygons for each target time period (1990, 2000, 2010) showing the urban extent during that time period. See Table A4.1 for information about the imagery files used in classification. Influence of Existing Urban Areas • Primary input data sets: urban extent delineations from previous step. • Modeling steps: –– Concentric zones were defined around the most recent urban extent at distances of 5, 10, 25, and 60 cells (some cities used 40 cells as the outer extent). –– Cells in each zone were weighted less the further away from the existing urban areas they fell. These weights were 100 for the closest ring, 75 for the next closest ring, 50 for the next closest, and 25 for the outer ring. • The final output of this step is the urban influence raster. Influence of Roads • Primary input data sets: Open Street Map data from 2011 (Open Street Map, 2011). All roads were considered regardless of type, except for Capetown, Durban, and Johannesburg, where road data comes from the Africa Infrastructure Country Diagnostic (AICD N.D.). • Modeling steps: –– The influence of roads was determined by the location of existing roads, the cells immediately adjacent to existing roads, and cells within a certain buffer distance of existing roads. –– Road cells themselves were defined as unsuitable for urbanization. –– A buffer distance of 15 cells was used to define near-road areas. • The final outputs of this stage were three rasters—roads, next-to-roads, and near-roads. Influence of Slope • Primary input data sets: 90-meter void-filled Shuttle Radar Topographic Mission (SRTM) digital elevation model developed by the Consultative Group on International Agricultural Research (CGIAR) (Jarvis et al., 2008), except for Accra, where HydroSHEDS 90-meter void-filled SRTM data (Lehner et al., 2008) were used. • Modeling steps: –– Slope was calculated from the elevation data and resampled to model resolution.