The gravity model of migration is a model in
urban geography
Urban geography is the subdiscipline of geography that derives from a study of cities and urban processes. Urban geographers and urbanists examine various aspects of urban life and the built environment. Scholars, activists, and the public have ...
derived from
Newton's law of gravity
Newton's law of universal gravitation is usually stated as that every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the square of the dista ...
, and used to predict the degree of migration interaction between two places. Newton's law states that: "Any two bodies attract one another with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them."
Overview
When used geographically, the words 'bodies' and 'masses' are replaced by 'locations' and 'importance' respectively, where importance can be measured in terms of population numbers, gross domestic product, or other appropriate variables. The gravity model of migration is therefore based upon the idea that as the importance of one or both of the location increases, there will also be an increase in movement between them. The farther apart the two locations are, however, the movement between them will be less. This phenomenon is known as
distance decay Distance decay is a geographical term which describes the effect of distance on cultural or spatial interactions. The distance decay effect states that the interaction between two locales declines as the distance between them increases. Once the d ...
.
The gravity model can be used to estimate:
*
Traffic flow
In mathematics and transportation engineering, traffic flow is the study of interactions between travellers (including pedestrians, cyclists, drivers, and their vehicles) and infrastructure (including highways, signage, and traffic control dev ...
* Migration between two areas
* The number of people likely to use one central place
The gravity model can also be used to determine the sphere of influence of each central place by estimating where the breaking point between the two settlements will be. An example of this is the point at which customers find it preferable, because of distance, time and expense considerations, to travel to one center rather than the other.
The gravity model can be used to measure accessibility to services (e.x., access to health care). A special case of gravity model is the
two-step floating catchment area method The two-step floating catchment area (2SFCA) method is a method for combining a number of related types of information into a single, immediately meaningful, index that allows comparisons to be made across different locations. Its importance lies in ...
(2SFCA), which is popular in health care research.
The gravity model was expanded by
William J. Reilly in 1931 into
Reilly's law of retail gravitation to calculate the breaking point between two places where customers will be drawn to one or another of two competing commercial centers.
[Reilly W.J. (1931). ''The Law of Retail Gravitation''. New York: Knickerbocker Press]
Opponents of the gravity model explain that it can not be confirmed scientifically, that it's only
based on observation. They also state that the gravity model is an unfair method of predicting movement because its biased toward historic ties and toward the largest population centers. Thus, it can be used to perpetuate the status quo.
See also
*
Gravity model of trade
The gravity model of international trade in international economics is a model that, in its traditional form, predicts bilateral trade flows based on the economic sizes and distance between two units. Research shows that there is "overwhelming ev ...
– uses the same approach for modelling trade flows between two locations, also discusses variants of the initial gravity model
*
Radiation law for human mobility
The radiation law is way of modeling human mobility (geographic mobility, human migration) and it gives better empirical predictions than the gravity model of migration which is widely used in this subject.
Intercity mobility
Waves of migration ...
*
Heuristic
A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be optimal, perfect, or rational, but is nevertheless sufficient for reaching an immediate ...
References
Literature
*
* Rodrigue, J.-P., Comtois, C., Slack, B. (2009). ''The Geography of Transport Systems''. London, New York: Routledge. .
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Urban geography