Maps constitute an intrical part of the types of resources used for genealogical and historical research. The following websites represent just a few of the many places to find map content online:
Historical
Maps (Library of Congress)
David Rumsey Map Collection
Historical Map Archives (Univ. of Alabama)
Perry-Castañeda Library Map Collection (Univ. of Texas at Austin)
Counties: Boundary Formation/History
Atlas of Historical County Boundaries
Urban
Sanborn Fire Insurance Map Collection (Lib. of Congress)
Geography / Geology
Atlas of the Historical Geography of the United States
The National Map (U.S. Geological Survey)
USGS – Streamer
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