1919 Official map of American and Canadian airways and aerial mail routes, by the Aeronautic Maps Association, from the Digital Commonwealth - commonwealth 7h149w33d. (<a rel="nofollow" class="external free" href="https://www.digitalcommonwealth.org/search/commonwealth:7h149w33d">https://www.digitalcommonwealth.org/search/commonwealth:7h149w33d</a>)
Aeronautic Maps Association
Digital Commonwealth