In 1878, the 69th mile post of the north-south boundary between the Wyoming and Dakota Territories, lying in 104°03’W longitude, was selected to become the initial point for the Black Hills Principal Meridian. The meridian extended the length of the existing territory boundary, and the principal base line, lying in 44° north latitude, was extended eastward from the initial point to the western boundary of range 79 West of the Fifth principal meridian survey. The Black Hills principal meridian and base line control all land surveys in the state of South Dakota north and west of White River, and west of the Missouri River, to the north and west boundaries of the Lower Brule Indian Reservation, and to the west boundary of the lands controlled by the Fifth Principal Meridian. Townships are run east of the Black Hills meridian, and ranges are run both north and south of the Black Hills principal base line.
