Nell's topographical map of the state of Colorado.
MLA
Nell, Louis. Nell's Topographical Map of the State of Colorado. Kendrick Book & Stationery Co., 1903. https://collections.library.yale.edu/catalog/15818421.
APA, 6th edition
Nell, L. (1903). Nell's topographical map of the state of Colorado. [Topographic maps]. Kendrick Book & Stationery Co., https://collections.library.yale.edu/catalog/15818421.