What History Pin does for historical photographs, HyperCities [ucla.edu] makes possible for geographic maps: seamlessly merging the historical representations of the city in their current situation, and thus connecting the digital archives, maps, and stories with the physical world.
A HyperCity is a real city overlaid with a large array of geo-temporal information, ranging from urban cartographies and media representations to family genealogies and the stories of the people and diverse communities who live there. The service now exists for the cities of Los Angeles, New York, Chicago, Rome, Lima, London (check out John Snow's cholera map!), Ollantaytambo, Berlin, Tel Aviv, Tehran, Saigon, Toyko, Shanghai, and Seoul, and will broadened in the future.
More information is available at the main HyperCities website.