This map was obtained from an edition of the National Atlas of the United States. Like almost all works of the U.S. federal government, works from the National Atlas are in the public domain in the United States.
Online access: NationalAtlas.gov | 1970 print edition: Library of Congress, Perry-Castañeda Library

This work is in the public domain in the United States because it is a work prepared by an officer or employee of the United States Government as part of that person’s official duties under the terms of Title 17, Chapter 1, Section 105 of the US Code. Note: This only applies to original works of the Federal Government and not to the work of any individual U.S. state, territory, commonwealth, county, municipality, or any other subdivision. This template also does not apply to postage stamp designs published by the United States Postal Service since 1978. (See § 313.6(C)(1) of Compendium of U.S. Copyright Office Practices). It also does not apply to certain US coins; see The US Mint Terms of Use.