Geographically referenced data consist of any type of measurement or observation, whether analog or digital, which have a known distribution across the surface of the ground, and hence can be presented as a map. Data of this sort are fundamental to all phases of mineral exploration and involve geological, geophysical and geo-chemical data along with topographic maps, aerial photographs, remote sensed imagery, mineral occurrence information, land use maps, drill hole location maps and so on. Computer software programs designed to store, manipulate and present geographical data are known as geographical information systems (GIS).