ways such flows transpire and the factors that interrupt or facilitate them. Scholars
of transportation and economic geography are especially focused on such move-
ment, but other geographers may also study flows. Movement occurs across space
because resources, people, and cultural attributes are unevenly distributed on the
landscapeof theEarth, and twolocationsmay sharecomplementarity, meaning
that exchange is advantageous to both. However, this movement is subject todis-
tance decay, which means that less interaction takes place as the distance between
two locations increases. This is due to the effect of thefriction of distance, a con-
cept that holds that costs, both material and in terms of time invested, increase
with distance. The larger the distance that separates two points, the greater the
costs associated with interaction between those points. Yet countless flows are
transpiring at any given moment on the Earth. Many flows are interrelated, and
in turn stimulate new movement. Trade flows, for example, may result in eco-
nomic expansion and greater employment opportunities in a location, and thereby
generatemigration, a flow of labor. Spatial interaction models provide a means of
representing and explaining the movementof people, commodities, technology,
and many other things through space. Many of these theoretical constructs were
developed in the 1930s and were elaborated on during thequantitative revolu-
tion.Christaller’sCentral Place Theorycan be considered a spatial interaction
model, and a number of others are in common use.
Perhaps the most frequently employed, and one of the best known, of the spatial
interaction models is the gravity model. This model applies the basic concept of
Newtonian physics regarding the force of gravity. Newton stated that the gravita-
tional force between two objects was equal to the mass of the first object multi-
plied by the mass of the second body, and then divided by the distance between
them squared. This model has been widely applied because it has been shown that
friction of distance between two places does not function in a linear way; rather,
like the physical force of gravity, it is inversely proportional to the square of the
distance separating them. In the gravity model employed in the social sciences,
Newton’s “force” is represented by the degree of flow, or interaction, between
two places, which are assigned a respective “mass” based on thepopulationof
each, or perhaps some other measure. The formula is:
The gravity model is highly useful in the study of transportation geography and
economic geography because it can be used with reasonable accuracy to predict
the degree of flow of traffic, goods, migration, and other items. Used in combination
Spatial Interaction Models 319