Tripoint - Minnesota, North Dakota, and South Dakota - Tripoints and Multipoints on Waymarking.com

The tri-point witness monument shown on the USGS topo map is easy to reach by farm roads near White Rock, SD. With a 4WD vehicle you can get quite close to it. There is some confusion, however, as to the exact location of the tri-point. The monument's E face contains an inscription stating that the tri-point lies 9 chains (594 feet) to the E. This corresponds to a position in the Bois de Sioux River, as shown on the following carefully measured map fragment (The intermediate tick mark corresponds to 594 feet.)

The map shows the tri-point only 80 feet E of the monument. We carefully measured the position of the USGS tri-point using electronic and paper copies of the map to obtained coordinates. We navigated to that position using GPS and found a partially exposed large rock, to which we added several smaller stones to form an embryonic cairn. We paced off a distance of 135 feet from the cairn to the witness monument. The 55 foot discrepancy between the map measurement and the paced distance is just barely within the 18 meter inherent GPS error and close to the 40-foot map accuracy standard used on the topo map, so our cairn is probably between 15 and 95 feet too far E.

Our best guess is that the inscription on the monument is correct (i.e., the tri-point is in the river). Historically, the river was the boundary between Minnesota and the Dakota Territory. We find it remarkable, though, that the tri-point presently is in the river because the river supposedly has been straightened and re-routed since the boundary was established.