The University of Portsmouth has launched a new online project with a title that refreshingly describes its purpose exactly: Old Maps Online. The site has collected links, locations, and metadata for over 60,000 historical maps, but the clever bit is that the interface for accessing them is built with MapRank Search software, which overlays each map's area on top of a standard Google Maps API. To find any map that contains a certain area, just search or zoom to the place you'd like and find lists of available maps on the right. Once you've located the one you'd like to see, a pop-up provides more context as well as a link to the map's host site, where you can see it in full. there's also a timeline slider up top so you can focus your search on just the era you're looking for — the earliest map in the database comes from none other than Gerhard Mercator himself, dated 1551 and courtesy of the Moravian Library.
Current contributors include the British Library, the David Rumsey Collection, and others, which together comprise the "broadest" collection of maps available online. Plans are in the works to double the number of maps in the database by the end of 2012, with contributions from the New York Public Library, Harvard University, and several others lined up.