A law is a general formula expressing a constant or usual relation between phenomena; laws brought to the fore by science of nature apply in physical geography; in human geography, laws are the necessary reference for the explanation, whether the object under study conforms itself to it or deviates from it (notions of exception, of residual with regard to a model); the term law is often considered as too strong and replaced by rule, or regularity, as the observed relations do not reflect the precision of physical laws (see the controversy about determinism); laws however express the existence of determinations in geography, even if these are not known or measured with precision, and if a more or less significant part of chance and uncertainty plays a role in their effects.
Several types of laws are used in geography. The distribution of certain geographical phenomena shows a regular form, meaning that it can be systematically found from one place to another or in different regions; they are sometimes described by means of mathematical laws (functions), such as the Poisson law which gives a good account of the spatial distribution of objects scattered inside territorial cells, the lognormal law or Pareto law which is largely resembled by all size distributions of geographical objects such as industrial plants, firms, cities, States...
The term law is also used in order to designate behavioural rules rather invariant in time or space such as the "law of least effort” formulated by Zipf, which shows that on average the shortest path or the closest proximity have been and still are favoured when locating human activities.
The term models is preferentially used for specifically geographical laws that rule the organisation of space: the law of formation of flows in function of the masses in presence and of their spacing, which allows predicting importance and configuration of exchanges or interactions is called gravity model, the law (derived from the former) of concentric organisation of the types of land uses in function of distance to a centre is illustrated by the models of von Thünen or Alonso, it may be talked about the law or model of spatial diffusion of epidemics or innovations.
See also: Nomothetism