MLA

Sayer, Robert, 1725-1794. A new map of the Northern States containing the Kingdoms of Sweden, Denmark, and Norway, with the western parts of Russia, Livonia, Courland &ca.. October 10th, 1788.. https://collections.library.yale.edu/catalog/2046634.

APA, 6th edition

Sayer, Robert, 1725-1794. (October 10th, 1788.)[A new map of the Northern States containing the Kingdoms of Sweden, Denmark, and Norway, with the western parts of Russia, Livonia, Courland &ca.]. https://collections.library.yale.edu/catalog/2046634.