In 1872, the U.S. Army Corp of Engineers began the immense project of mapping the United States “West of the 100th Meridian” on a large scale, a program expected to take 15 years. Lieutenant George Wheeler, who had run surveys in Arizona and New Mexico in 1871, was chosen to head this project, leading to yearly surveys of the American West between 1872 and 1878, at which time Congress stopped appropriations because of cost and possible duplication. These maps are from Wheeler’s atlas of sheets showing the incredible work which resulted. The detail is amazing, especially given the rough terrain depicted, with that topography shown graphically with hatchuring. The routes of various expeditions noted.