Map Overlay Tool

How does the Map Overlay Tool work?

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.

Why aren't all maps in the Free Library Map Collection available in the Map Overlay Tool?

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.

Why is the Map Overlay Tool useful?

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).

What are some additional resources I can look into if I want to learn more about historical GIS (Geographic Information Systems)?

Knowles, Anne Kelly, Hillier, Amy. Placing History: How Maps, Spatial Data, and GIS are Changing Historical Scholarship, ESRI Press: Redlands, CA. 2008 Knowles, Anne Kelly, ed. Past Time, Past Place: GIS for History, ESRI Press, Relands, CA. 2002. Chapter 1. List of GIS-related publications complied by Washington University, St. Louis

Digital Maps
of Philadelphia

Historical Maps

Base Maps

Digital Mapsof Philadelphia

Historical Maps

Base Maps

Digital Maps
of Philadelphia