Fourth Principal Meridian
The fourth principal meridian, set in 1815, is the principal meridian for land surveys in northwestern Illinois and west-central Illinois, and its 1831 extension is the principal meridian for land surveys in Wisconsin and northeastern Minnesota. It is part of the Public Land Survey System that covers most of the United States.
The fourth principal meridian begins at a point on the west bank of the Illinois River in Schuyler County, Illinois. The fourth principal meridian's baseline, sometimes called the Beardstown baseline, runs west from this initial point. The meridian and this baseline governs surveys in Illinois that are west of both the Illinois River and the third principal meridian.
The Illinois Department of Transportation 2003 Survey Manual gives the point as 40°0′50″N 90°27′11″W / 40.01389°N 90.45306°WCoordinates: 40°0′50″N 90°27′11″W / 40.01389°N 90.45306°W and notes that the meridian is an extension of the line north from the mouth of the Illinois River near Grafton, Illinois.
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