The app is based on "Geometrischer Plan von Frankfurt am Main" (Geometric map of Frankfurt am Main) of August Ravenstein. Published by Verlag des geographischen Instituts zu Frankfurt am Main in the year 1862.
The historical map can be used offline, i.e. without an Internet connection. A today's map (the OpenStreetMap) can be displayed to compare the historical map with the present situation.
If you are inside the area of the historical map the app determines your location and shows it on the map.
With 'Frankfurt historically pro' you can enjoy a completely ad-free version of the app and support the development of this and other projects.