The gravity model of migration is a model in urban geography derived from Newton’s law of gravity, and used to predict the degree of 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.”
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.
The gravity model can be used to estimate:
- Traffic flow
- Migration between two areas
- The number of people likely to use one central place
It is a model to predict movement of people, information, and commodities between cities and even continents.
The gravity model takes into account the population size of two places and their distance. Since larger places attract people, ideas, and commodities more than smaller places and places closer together have a greater attraction, the gravity model incorporates these two features.
The relative strength of a bond between two places is determined by multiplying the population of city A by the population of city B and then dividing the product by the distance between the two cities squared.
Population 1 x Population 2
Thus, if we compare the bond between city A and City B, we first multiply their populations and then we divide the ressult by the distance squared Therefore, the bond between New York and Los Angeles is greater than that of El Paso and Tucson!
While the gravity model was created to anticipate migration between, it can also be used to anticipate the traffic between two places, the number of telephone calls, the transportation of goods and mail, and other types of movement between places. The gravity model can also be used to compare the gravitational attraction between two continents, two countries, two states, two counties, or even two neighborhoods within the same city.
Some prefer to use the functional distance between cities instead of the actual distance. The functional distance can be the driving distance or can even be flight time between cities.
It 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.
Critiques of the gravity model argue 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.