Gouverneur K. Warren, Map of the Territory of the United States from the Mississippi to the Pacific Ocean (1857)

This map by Gouverneur K. Warren, a civil engineer in the Army Corps of Topographical Engineers, was one of the first detailed maps of all the land west of the Mississippi, marking an important step toward building the transcontinental railroad route. According to James R. Akerman and Robert W. Karrow, Jr., Warren had to take multiple and sometimes conflicting regional maps and compile them into a comprehensive view of the west. In the process, he managed to standardize longitude on the map, making this one of the most important and foundational maps of the West in the 19th century.