The Montana principal meridian and base line were established on an initial point in the southwestern part of the state, approximately ten miles southwest of the headwaters of the Missouri River, near Willow Creek. The meridian lies in 110° 39’ 33”W longitude, and the base line lies in 45°47’ 13”N latitude. These lines govern all PLSS surveys in the state of Montana. Ranges were run both east and west of the Montana principal meridian, and survey townships were run both north and south of the Montana principal base line. The initial point was set in 1867, and is still marked by the brass cap placed there in 1922 by the U.S. Land Office.
