The boundary between the Wyoming and Dakota Territories, a north/south line that divides these states today, was established in 1877. In 1878, Mile 69 on the line was selected to be the Initial Point for the survey of western South Dakota and its Black Hills, running the meridian up the existing territory boundary, and extending a baseline eastward. This was heavily contested territory, still to this day claimed by the Sioux Indians. 1877 was the year that Crazy Horse surrendered and was killed, and the year the treaty taking the Black Hills from the Sioux was signed.
The meridian is now the state line between Wyoming and South Dakota. The baseline, from Mile 69, can be seen running eastward from the Initial Point.
Most of the western half of South Dakota was surveyed off the Black Hills Principal Meridian (in red).