with various statistical techniques, the model provides substantial insight into the
workings of networks through space and what factors affect their effectiveness. It
assumes that the closer places are to one another, the greater the level of interaction
between them, an assumption repeatedly borne out in studies of actual flows.
An adaptation of the gravity model is Reilly’s Model of Retail Gravitation, a
somewhat more sophisticated spatial interaction model that attempts to establish
boundariesof interaction betweenregions, rather than the interaction between
two places. Reilly used retail sales data from counties in Texas to develop his
model. He was interested in discovering the breaking point, or boundary, between
two sales regions. Reilly assumed that the larger the population in a city, the
greater the retail sales area it wouldcommand. But where would the boundary
be between two cities of unequal population? Reilly’s model predicts the location
of this border through the equation:Here again, population is the equivalent of mass in Newton’s theory. In the case
where two cities would have the same population, the breaking point would appear
exactly half way between them. Reilly’s model has been used not only by eco-
nomic geographers in academic studies of sales areas and interaction but has
applications in business and marketing. Furthermore, it may be applied beyond
retail sales, to find boundaries between media markets, zones of loyalty to sports
teams, and many other phenomena.Squatter Settlements
Areas of dense, low-quality housing around many large cities in the developing
world, usually formed by illegal occupation, or “squatting,” of unclaimed land.
Squatter communities are created by themigrationof poor, rural residents to cities
who are drawn there by thepush-pull concept, especially the allure of higher-
paying jobs. Over the past forty years many urban areas in the developing world
have witnessed a surge in new arrivals from thehinterlandthey serve, and the
supply of local housing has simply been unable to keep pace with the increase in
population. New residents have therefore been forced to build ramshackle sheds
on unclaimed or public land. Globally, an enormous number of people live in320 Squatter Settlements