You will need:
Author/Cartographer Surname, Initials or organisation (Year) Title or description of map, scale. [Map]. Place of Archive: Name of Archive. Catalogue/ reference number.
Wyld, J. (1850) London and its environs: levels taken by order of the commissioners of the sewers, 1:17,400. [Map]. London: British Library. Reference number: BLL01016133799.
In-text citation: (Wyld, 1850)