Traditional raster and vector data models cannot easily represent the more complex aspects of geographic information, such as temporal change, uncertainty, three-dimensional phenomena, and integrated multimedia. A variety of models have been proposed to represent these complexities, including both extensions to existing models and software, and entirely new models and software. During the 1990s, work in this area was largely experimental, but many solutions are now available to practitioners in commercial and open source software. The data models in this unit are based on concepts discussed in Knowledge Area CF Conceptual Foundations.