The basic question here is you need to determine whether a single map has moved into the public domain. If the map was created before Jan. 1, 1923, it is in the public domain and may be digitized. For any maps created between Jan 1, 1923 and Jan. 1, 1978, different rules apply. If the map was published between 1923-1963, it must contain a copyright notice or symbol and have been registered. For copyright protection to have been continued, the registration must have been renewed 28 years later. Maps created between 1963 and 1978 are likely still protected due to copyright extension -- and anything after 1978 is copyrighted with or without any kind of symbol -- and registration was done away with as a requirement in 1991.
To determine initial status and renewal -- you have to look at the Copyright Office's Copyright Registration and Renewal Records. These have been largely digitized and are available online at a site maintained by Penn (http://onlinebooks.library.upenn.edu/cce/ ) and at Internet Archive (http://archive.org/details/copyrightrecords)
Some additional resources that may help you:
Article on digitizing Sanborn Maps http://www.dlib.org/dlib/july02/arlitsch/07arlitsch.html
Short work on determining public domain status: http://homepages.law.asu.edu/~dkarjala/opposingcopyrightextension/publicdomain/searchc-r.html
Here is the Copyright Office's instructions on how to investigate the copyright status of a work (I recommend starting on p. 6 "Works First Published Before 1978") http://www.copyright.gov/circs/circ22.pdf
