Gravity Model – resembles Isaac Newton’s formula for grativational attraction between any two ce...
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Gravity Model – resembles Isaac Newton’s formula for grativational attraction between any two celestial. masses. Population size and distance are used to explain the interaction flow Iij, between origin I and destination j. Gravity model allows both size and distance to vary simultaneously:.

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Slide1 l.jpg

Gravity Model – resembles Isaac Newton’s formula for grativational attraction between any two celestial

masses.

Population size and distance are used to explain the interaction flow Iij, between origin I and destination j.


Gravity model allows both size and distance to vary simultaneously l.jpg
Gravity model allows both size and distance to vary simultaneously:

  • Iij=k PiPj

  • B

  • dij

  • Where Iij=predicted interaction between origin I and destination j.

  • K=a scaling constant

  • Pi=a measure of size, usually population for origin i.

  • Pj=a measure of size, usually population for destination j.

  • B= am exponent which adjusts for the rate of distance decay unique to the type of interaction being measured.


This model can be modified to model all types of spatial interaction l.jpg
This model can be modified to model all types of spatial interaction.

  • The mass or size variables in the numerator of the fraction will have a positive relationship with spatial interaction. Thus, as population of a state increases, both for origins and destinations, the interaction between them increases. Distance, being denominator, will be negatively, or inversely related to interaction. Interaction decreases as distance increases.

  • The other two factors are constants calibrated statistically to produce the most realistic levels of interaction between places.


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