MLA

Magnetic Declination : Isogonic Lines for the Year 1870 / Drawn by A. Lindenkohl ; Chas. G. Krebs, Lith. U.S. Coast Survey Office, 1866. https://collections.library.yale.edu/catalog/15603706.

APA, 6th edition

Magnetic Declination : Isogonic lines for the year 1870 / Drawn by A. Lindenkohl ; Chas. G. Krebs, Lith. (1866). [Maps]. U.S. Coast Survey Office, https://collections.library.yale.edu/catalog/15603706.