David Rumsey's amazing collection of historical maps has been available as a layer on Google Earth since 2006. Now they are available on Google Maps!
The David Rumsey Google Map Collection contains an amazing 120 historical maps overlaid on Google Maps. For, example, the screen shot above shows an 1832 map of London overlaid on the current Google Map of London (it also reveals that my house in east London was been built on top of a marsh). Each map's transparency can be adjusted with a slider button, so it is possible to find a location on the present Google Map before overlaying the historical map.
Highlights of the new maps include the first accurate survey of Yosemite Valley, California, from 1883, maps of Los Angeles, San Francisco, Seattle, New York, Washington DC, Denver, Chicago, Moscow, St. Petersburg, Calcutta, Lima, Paris, Madrid, Rome, Beijing, Tokyo, Kyoto, and more – all from times ranging from the 18th to the 20th centuries.
Via: Google LatLong: New Rumsey Historical Maps and new ways to view them