APA (7th ed.) Citation

Great Britain. Ordnance Survey. (1916). Map of the Somme area. Geographical Section, General Staff.

Chicago Style (17th ed.) Citation

Great Britain. Ordnance Survey. Map of the Somme Area. [London?]: Geographical Section, General Staff, 1916.

MLA (8th ed.) Citation

Great Britain. Ordnance Survey. Map of the Somme Area. Geographical Section, General Staff, 1916.

Warning: These citations may not always be 100% accurate.