S.D. Codified Laws § 7-1-7

Beginning on the boundary line between the states of South Dakota and North Dakota where the eighth guide meridian west, intersects the same, thence west and along said state line to the northwest corner of township one hundred twenty-eight north of range sixty-five, west of the fifth principal meridian; thence south along the range line between ranges sixty-five and sixty-six west, to the fifth standard parallel north; thence east and along said fifth standard parallel to said eighth guide meridian; thence north along said eighth guide meridian to the place of beginning.