"We believe these maps have no known US copyright restrictions," explains a blog post announcing the wholesale release of the library's map collection. "It means you can have the maps, all of them if you want, for free, in high resolution. We’ve scanned them to enable their use in the broadest possible ways by the largest number of people." The NYPL is distributing the maps under Creative Commons CC0 1.0 Universal Public Domain Dedication, which means you can do whatever you want with the maps."
Link to Original Source