Historical maps are geographical representations made with the intention to represent spatial facts over time. Historical maps are generally considered valuable documents not just because of their historical value but also because most of them also are artistic representations by themselves. From a cartographical point of view, differentiation between historical maps and actual maps is mainly based on the advances in the history of Cartography, so once one disruptive advance in the map making process appears, maps created with previous techniques (and with some artistic or historical value) are usually considered as historical, such as ancient paper-based maps or old sea maps, for instance. Techniques such as scanning or photography can make ancient maps publicly available by converting hard-copy maps to digital ones. Once an historical map is digitised, the next step is to georeference it, which is the process of specifying and relating points of the digitalised map to actual coordinates in a geographic reference system. Because of its archival value and interest, historical maps are adequately preserved - following specific conditions - by map libraries, map societies or museums. Since digital methods and techniques have been replaced over time by new technological advances, first digitally created maps could be also considered historical, not because of its content, but of the techniques used to produce it.