A striking aspect of today's cities is their rapid outward thrust. Urban land values being what they are, cities gobble up farmland at the rate of about 400,000 acres a year. At the same time, vacant
DATASET: Alpha version 2000 and 2010 estimates of numbers of people per grid square, with national totals adjusted to match UN population division estimates (http://esa.un.org/wpp/) and MODIS-derived