Time–space geography provides a method of mapping spatial movements through time. Hägerstrand (in T. Carlstein 1978) expressed the time and space needed for events as a web model, based on four propositions:

• • space and time are scarce resources which individuals draw on to achieve their aims

• • achieving an aim is subject to capability constraints, coupling constraints, and authority constraints

• • that these constraints interact to demarcate probability boundaries

• • that choices are made within these boundaries.

Ahmed and Miller (2007, J. Transp. Geog. 15) acknowledge Hägerstrand’s conceptual framework, but argue that ‘rigorous, analytical definitions of basic time geography entities and relationships do not exist’. Accordingly, they propose ‘analytical formulations for basic time geography entities and relations, specifically, the space–time path, prism, composite path-prisms, stations, bundling, and intersections’. Raubal et al. (2004) Geografiska B 86 also propose a spatio-temporal theory of location-based services, which they claim to be closer to the individual user and more plausible with respect to her or his daily life. See also Miller (2005) Geog. Analysis 37, 1. A time–space prism is a representation of the constraints limiting the time within which the individual can act.