The Free Library's Map Overlay Tool is designed to bring history to the present! Historic maps from the Free Library Map Collection can be layered on top of a current road map, terrain map, or hybrid (satellite/road) map. Compare the similarities and differences between the old and new maps by adjusting the "visibility" bar, and see how Philadelphia has changed over time!
Historic maps are scanned by a skilled image technician. Maps are then uploaded and cataloged for viewing in our digital collection. Technicians then georeference (assigning geographical locations to the historical map that align with the modern map) the historical map to a modern map so that it is searchable by location or address.
The Free Library Map Collection has over 130,000 maps--georeferencing each one would take a lot of time! Georeferencing is as much an art as it is a science. Technicians often have to modify the historical map in order for it to align properly to the modern map. They have to ensure the proper scale is maintained and that distortion is limited while also identifying enough reference points to align the two maps.
It's a unique way to study Philadelphia's development! You can compare and contrast historical Philadelphia with modern Philadelphia to see what's changed and what's stayed the same (examples: street names and street placement, natural features like streams and shorelines, and natural or artificial boundaries).