Public Land Survey System
Fourth Principal Meridian
The Fourth Principal Meridian was set in 1815, and is the principal meridian governing all land surveys in northwestern Illinois and west-central Illinois. It was extended in 1831 to become the principal meridian for land surveys in Wisconsin and northeastern Minnesota. The Initial Point for the Fourth Principal Meridian lies on the west bank of the Illinois River in Schuyler County, Illinois at 40°0'50"N 90°27'11"W. From this point, the meridian runs due north until it intersects territory controlled by the Fifth Principal Meridian. The Fourth Principal Meridian's principal base line south of Wisconsin, sometimes called the Beardstown Baseline, runs west from the Initial Point. The Fouth Principal Meridian and the Beardstown Baseline control surveys in Illinois that are west of both the Illinois River and the Third Principal Meridian, and south of the Illinois-Wisconsin border.