The New York Public Library’s Map Division has released more than 20,000 historic maps as high resolution downloads under a Creative Commons CCO 1.0 Universal Public Domain Dedication. Their collection is especially good for New York City and New York State maps. The easiest place to search for the maps is through the NYPL’s Digital Collections portal. Once you find maps you want to download, you can then go to the NYPL’s Map Warper and search by Image ID. You will need to create a Map Warper account.
The Map Warper is a tool that allows users to stretch images of scanned maps onto digital maps to align historical geography with the present. When a map is scanned, it exists only as an image file: "warping" the map gives it geographic meaning in the digital world. You can read more about the Map Warper and its uses in this blog post by the NYPL's Geospatial Librarian. Students in Pratt's School of Information and Library Science made a brief video tutorial that will help you navigate the Map Warper.
You can learn more about the NYPL's Map Division and the resources available to you here.
The Library of Congress has the biggest and most comprehensive collection of cartographic materials in the world: “over 5.2 million maps, including 80,000 atlases, 25,000 geospatial datasets, 6,000 reference works, numerous globes and three-dimensional plastic relief models, and a large number of cartographic materials in other formats,” according to their website. The Librar of Congress’ collections include maps and geospatial information about the United States and the rest of the world. Many of the Library of Congress’ collections have been scanned and are available for free online.
The best way to search the collection is from www.loc.gov/maps.You can limit your search to items that are available online, and there are good options to refine your search – original formats, date, sites and collections, contributors, subject, locations, and languages. The scanned maps can be downloaded in several formats and sizes, and researchers can zoom in on the website to see fine details.
In addition to searching the map collection, researchers can look for maps through Library of Congress' subject guides.
The University of Texas’ Perry-Castañeda Library Map Collection offers an extensive list of maps available online including:
Polar and ocean maps
Outline and unlabeled maps
The U.S. Geological Society’s National Map is a collaborative program among USGS and Federal, State, and local government agencies to offer topographic information for the United Sates. The National Map includes aerial photographs, elevation, geographic names, hydrography, boundaries, transportation, and land cover. Within The National Map’s viewer, you can bring in other types of geographic information, or you can add National Map data into a Geographic Information System.
The David Rumsey Map Collection is a rich source for images of historical maps. The private collection focuses on rare 18th and 19th century maps of North and South America. Over 53,000 digitized maps are available online.