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# gravity model resembles isaac newton s formula for grativational attraction between any two celestial - PowerPoint PPT Presentation

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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### 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:

• 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.

• 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.