Digital Maps

U. S. Geological Survey-Maps, Imagery and Publications
http://www.usgs.gov​
The USGS is a science organization that provides impartial information on the health of our ecosystems and environment, the natural hazards that threaten us, the natural resources we rely on, the impacts of climate and land-use change, and the core science systems that help us provide timely, relevant, and useable information.

Library of Congress Geography & Map Reading Room
http://www.loc.gov/rr/geogmap/guides.html
The Geography and Map Division of the Library of Congress provides cartographic and geographic information for all parts of the world to the Congress, Federal agencies, state and local governments, the scholarly community, and to the general public. It is the largest and most comprehensive cartographic collection in the world, numbering over 5.2 million maps, including 80,000 atlases, 6,000 reference works, numerous globes and three-dimensional plastic relief models, and a large number of cartographic materials in other formats, including electronic.

Perry-Casteñeda Library Map Collection
http://www.lib.utexas.edu/maps
A large collection of public domain country, city and thematic maps, including Army Map Service maps, presented as gif or jpeg files, provided by the University of Texas.

David Rumsey Map Collection
http://www.davidrumsey.com
The historical map collection has over 27,000 maps and images online. The collection focuses on rare 18th and 19th century North American and South American maps and other cartographic materials. Historic maps of the World, Europe, Asia, and Africa are also represented.

Mapco
http://mapco.net/index.htm
The MAPCO website is a philanthropical project, designed to raise awareness and appreciation of history using old maps and photographs. The website displays many high quality historical maps and views, with an emphasis on London, U.K., and Australia. The MAPCO website is a free access site, and will remain so.  This site is intended primarily as a research aid for genealogical and historical researchers. The maps, prints and views on the MAPCO website are displayed free of charge for the use of students, historians and genealogists, to assist with heritage and family history research.

New York Public Library Digital Collections – Maps & Atlases
https://digitalcollections.nypl.org/collections/lane/maps-atlases
This large and prestigious institution now offers free access to thousands of digitized items with new materials added daily.  Among their offerings are atlases of the United States that include hundreds of images for Erie County.