  Predicting urban growth is important for practical reasons, and also for the
challenge it presents to theoretical frameworks for cluster dynamics. Recently,
the model of diffusion limited aggregation (DLA) has been applied to describe
urban growth, and results in tree-like dendritic structures which have a core
or ``central business district'' (CBD). The DLA model predicts that there
exists only one large fractal cluster that is almost perfectly screened from
incoming ``development units'' (people, capital, resources, etc), so that
almost all the cluster growth occurs in the extreme peripheral tips. Here we
propose and develop an alternative model to DLA that describes the morphology
and the area distribution of systems of cities, as well as the scaling of the
urban perimeter of individual cities. Our results agree both qualitatively and
quantitatively with actual urban data. The resulting growth morphology can be
understood in terms of the effects of interactions among the constituent units
forming a urban region, and can be modeled using the correlated percolation
model in the presence of a gradient.
