This primary source comes from the Records of the office of the Chief of Engineers.
Full Citation: Map Showing the Extent of Surveys of the Territory of Utah, annotated with red flag showing Promontory Summit; 1856; New Mexico, Utah, California, Oregon, and Washington; 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/map-showing-extent-of-surveys-of-utah, April 26, 2024]