MLA

Section, Great Britain. War Office. General Staff. Geographical. Western Theatre of War. G.S.G.S., 1915. https://collections.library.yale.edu/catalog/15237917.

APA, 6th edition

Section, G. B. W. O. G. S. G. (1915). Western theatre of war. [Maps]. G.S.G.S., https://collections.library.yale.edu/catalog/15237917.