The gravity model of migration is a model, derived from Newton's law of gravity, is used to predict the degree of interaction between two places (Rodrigue et al. 2009, 216). 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 another 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 basic gravity model is as follows:
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Mij=k
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Pi*Pj |
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dij2 |
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where: Mij = gravity model prediction of migration between origin i and destination j Pi = population of origin State i Pi = population of destination State j dij = distance from origin i to destination j k = a constant that adjusts the gravity model estimates so that the total numbers of actual and estimated migrants are approximately equal |
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* In this experiment we will leave out the variable k from the original gravity model for simplicity.
The simplified version:
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Let's begin. Follow the instructions below and complete the tasks to the best of your ability:
Use this list of US city populations for the locations you choose.
Use the Google Maps Distance Calculator to determine distances between locations.
1. Practice: find the population totals for Fort Lauderdale and New York City, then determine the distance between them (in miles).
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Plug in the numbers into the Gravity Model formula to calculate the intensity of interaction between the two locations (this number does not mean anything specific since we have left out any variables that would adjust this number to any meaningful measurement) (your answer should be around 1,000,000 to 1,250,000). |
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Find the population totals for Fort Lauderdale and Miami, then determine the distance between them. Plug these number into the Gravity Model formula (your answer should be around 110,000,000 and 150,000,000). |
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Calculate the difference of migration intensity between Fort Lauderdale and Miami, and between Fort Lauderdale and New York City by dividing the smaller number into the larger one (your answer should indicate 80 to 110 times greater). |
2. Now ... choose THREE other pairs of US cities (and find the populations of each); e.g., Tampa Bay and Boston). Calculate the migratory intensity (gravity model formula) between the chosen pairs of cities. Complete the following tasks:
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Identify the cities you chose. |
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List the populations of the cities. |
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List the distances between the locations. |
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List the calculations of the levels of migration between the pairs of of cities. |
3. Answer the following questions:
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How might the calculations of distance you came up with differ from actual travel distances in real life? |
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What other factors might influence people to travel one city or another (other than distance decay)? |
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How accurately do you believe the gravity model formula predicts actual migratory patterns between places? Explain. |