The next models developed were the gravity model and the intervening opportunities model.

The most widely used formulation is still the gravity model.

The gravity model illustrates the macroscopic relationships between places (say homes and workplaces).

Limiting the usefulness of the gravity model is its aggregate nature.

The gravity model can be used to measure accessibility to services (e.g., access to health care).

The gravity model, in its basic form, predicts trade based on the distance between countries and the interaction of the countries' economic sizes.

This observation is embedded in the gravity model of trip distribution.

The gravity model estimates the pattern of international trade.

Using the gravity model, countries with similar levels of income have been shown to trade more.

The type of gravity model used for the Earth depends upon the degree of fidelity required for a given problem.