On http://www.lib.utexas.edu/maps/faq.html#3.html it says:
"Most of the maps scanned by the University of Texas Libraries and served from this web site are in the public domain. No permissions are needed to copy them. You may download them and use them as you wish.
A few maps are copyrighted, and are clearly marked as such. Any that are copyrighted by The University of Texas are subject to our Materials Usage Guidelines."