This primary source comes from the Records of the Office of the Chief of Engineers.
Full Citation: Contour Map of the United States; ca. 1885; United States; Civil Works Map File, 1818 - 1947; Records of the Office of the Chief of Engineers, Record Group 77; National Archives at College Park, College Park, MD. [Online Version, https://www.docsteach.org/documents/document/contour-map-of-the-united-states, May 3, 2024]