We propose and test a model that describes the morphology of cities, the sc
aling of the urban perimeter of individual cities, and the area distributio
n of systems of cities. The model is also consistent with observable urban
growth dynamics, our results agreeing both qualitatively and quantitatively
with urban data. The resulting growth morphology can be understood from in
teractions among the constituent units forming an urban region, and can be
modeled using a correlated percolation model in the presence of a gradient.
[S1063-651X(98)04012-4].