A critical look at representations of urban areas in global maps

According to the UN, the number of urban dwellers is expected to increase from roughly 3.2 billion today to more than 4.9 billion by 2030. An accurate and regularly updated estimate of the extent and spatial distribution of urban land is an important first step in our search for realistic responses to the ecological and social consequences of what promises to be the most rapid urbanization in world history. By employing circa-2000 satellite remote sensing imagery, geographic information systems, and census data, six groups from government and academia in both the EU and the US have created global maps that can be used to describe urban land. We compare these maps from global to sub-national scales, for the first time applying Discrete Global Grids to the problem of global-scale map comparison. Although most of these maps share common data inputs, they differ by as much as an order of magnitude in their estimates of the total areal extent of the Earth's urban land (from 0.27 to 3.52 million km(2)). A sub-national analysis of the spatial distribution of urban land reveals that inter-map correlations are highest in North America ((r) over bar = 0.90), intermediate in Europe, South and Central America, and Sub-Saharan Africa ((r) over bar= 0.78), and lowest in Asia ((r) over bar = 0.63). Across most regions, our analysis uncovers a degree of variance that is high enough to call into question the consistency of each group's approach to urban land, pointing to the need for both a common urban taxonomy and a global urban assessment effort.