A website called Georeferencer allows users to overlay historical maps on top of modern mapping data, giving a historical perspective on how cities developed over time. In a new version released last month, the tool is redesigned for use on tablets and mobile devices, allows collaborative work on maps and provides tutorials on how to use the software.
Users can zoom into areas on a modern map, select a historical map from a library of scanned documents, and then see how the area has changed over time. A transparency slider allows users to flip back and forth between the past and the present. Seattle shows land reclamation efforts of an area called Harbor Island. In one 1890 map of the city, Elliott Bay west of downtown Seattle is empty, while modern data shows Harbor Island, an artificial land mass completed in 1909.
A similar project can be spotted in San Francisco. A 1859 map of the area is notable for its lack of key areas, including Treasure Island, an artificial island built in 1937. Also missing are the Golden Gate and San Francisco–Oakland Bay Bridges. Users who notice an error can re-orient the map by selecting three points on each map to improve the precision of the map. The aim for the tool is researchers and geographic information systems (GIS), but the average user can look at how their favorite areas have changed over time.