The following is an extract from the New York Public Library's web site at http://www.nypl.org/blog/2014/03/28/open-access-maps:
The Lionel Pincus & Princess Firyal Map Division is very proud to announce the release of more than 20,000 cartographic works as high resolution downloads. We believe these maps have no known US copyright restrictions.* To the extent that some jurisdictions grant NYPL an additional copyright in the digital reproductions of these maps, NYPL is distributing these images under a Creative Commons CC0 1.0 Universal Public Domain Dedication. The maps can be viewed through the New York Public Library’s Digital Collections page, and downloaded (!), through the Map Warper. First, create an account, then click a map title and go. Here’s a primer and more extended blog post on the warper.
What’s this all mean?