In the Urban extents data, how were the urban areas identified?
The primary source of data for identifying urban areas is the Nighttime Lights data set from the Defense Meteorolocical Satellite Program (DMSP), see http://dmsp.ngdc.noaa.gov/html/download_Night_time_lights_94-95.html
DCW (Digital Chart of the World) urban areas were used as a secondary source. For Africa, additional information was used, primarily from Tactical Pilotage Charts (TPCs).