Ladd, R. S. 1. (1962). Maps showing explorers' routes, trails & early roads in the United States: An annotated list. Washington: For sale by the Superintendent of Documents, U.S. Govt. Print. Off..

Ladd, Richard S. 1899-. Maps Showing Explorers' Routes, Trails & Early Roads in the United States: An Annotated List. Washington: For sale by the Superintendent of Documents, U.S. Govt. Print. Off., 1962.

Ladd, Richard S. 1899-. Maps Showing Explorers' Routes, Trails & Early Roads in the United States: An Annotated List. Washington: For sale by the Superintendent of Documents, U.S. Govt. Print. Off., 1962.