<Li> The number of people likely to use one central place </Li> <P> 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 . </P> <P> The gravity model can be used to measure accessibility to services (e.g., access to health care). A special case of gravity model is the two - step floating catchment area method (2SFCA), which is popular in health care research . </P> <P> 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 . </P>

Who made the gravity model of spatial interaction