S.D. Codified Laws § 7-1-53

Beginning at a point where the one hundred second meridian of west longitude intersects the boundary line between the states of South Dakota and North Dakota, thence west and along said state line to the range line between ranges nine and ten east Black Hills meridian; thence south and along said range line to the third standard parallel north of the Black Hills base line; thence east and along said standard parallel to the said one hundred second meridian; thence north and along said meridian to the place of beginning.