The paper outlines the nature of the geographical information system that was developed for the BBC Domesday optical disc project. The successful exploitation of optical disc storage for geographical analysis and mappings dependent on the development of appropriate data structures that allow fastest possible data retrieval for geographical areas of interest. The authors describe how they achieved this objective such that a low powered BBC micro-computer is able to offer interactive colour mapping of over 20,000 variables for 33 different sets of geographical areas at ten different levels of resolution