Thus, from a fundamental focus on pure spatial form, attention turned to the movements of objects and to the passage
of events through space and time. Models to simulate urban growth and other phenomena as processes of spatial diffusion (see Diffusion: Geographical Aspects) (represent explicit attempts to mesh the time and space dimensions in geographical research. Significant statements advancing time-space perspectives are found in Ha$gerstrand's (1973) conceptualizations
of spatial diffusion and time-geography, Janelle's (1969) formulation of time±space convergence, and Harvey's (1990) notion of time±space compression.