Public domain material is that which is free of copyright. With rare exception all original works are copyrighted automatically by USA copyright law. An author is under no obligation to provide copyright notice and the absence of such notice should not be considered a release of rights. Explicit permission must be given or use of materials is a violation of copyright law.
For most works, "public domain" includes those for which the copyright has expired (70 yrs after the death of author), or the work was ineligible for copyright. The latter includes U.S. national government publications, such as from the USDA, USGS, NASA, NOAA, NSF, etc..
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