MLA

Section, Great Britain. War Office. General Staff. Geographical. Germany Ethnographical Map. War Office [G.S.G.S.], 1918. https://collections.library.yale.edu/catalog/15238237.

APA, 6th edition

Section, G. B. W. O. G. S. G. (1918). Germany ethnographical map. [Maps]. War Office [G.S.G.S.], https://collections.library.yale.edu/catalog/15238237.