MLA

Section, Great Britain. War Office. General Staff. Geographical. Skeleton Map of Western Europe (French 1/500,000) [Map]. War Office, Geographical Section, General Staff, 1914. https://collections.library.yale.edu/catalog/15259001.

APA, 6th edition

Section, G. B. W. O. G. S. G. (1914). Skeleton map of Western Europe (French 1/500,000) [map]. [Maps]. War Office, Geographical Section, General Staff, https://collections.library.yale.edu/catalog/15259001.