Maps pertaining to surface water (historically and currently) in Philadelphia, and the city's water supply and sewerage systems.  The site also provides links to many scanned, old atlases, topographical maps, etc. for Philadelphia (including some complete atlas volumes), nearby communities, and counties in southeastern Pennsylvania.

Aerial photography of  Philadelphia, the Delaware Valley, and the Mid-Atlantic region  produced by the Dallin areial Survey Co., 1924-1941.

Accessible via the Hagley Museum & Library's Hagley Digital Archives.

Key map showing which areas of Philadelphia are covered by which numbered, standard Sanborn Co. fire insurance map volumes for the city.

More-or-less essential starting point for using the Sanborn maps in printed or digital form.

Photos, info., compiled maps, scanned maps of Philadelphia region transit lines, especially trolleys.  Also has some info about Toronto and San Francisco areas.  An enthusiast website.

An occasional blog by Philadelphia Inquirer architecture critic Inga Saffron.  The page includes a link to her Phila. Inquirer weekly columns.