via GVRL (Gale Virtual Reference Library).

County soil surveys include a brief overview of regional geography.

The PASDA Imagery Navigator provides access to the PASDA aerial photography collection via a search interface that displays all data available for any place in the state. Most of this collection is aerial orthoimagery and provides download access as well as streaming data services.

The PAMAP program was a collaboration among state agencies to produce a collection of statewide high resolution digital orthoimagery and lidar elevation data.

This site shows changes in county boundaries over time.

This resource points to many county-level data sources.

Historical aerial photography from 1937 to the present.