The fourth principal meridian, set in 1815,[1] is the principal meridian for land surveys in northwestern Illinois and west-central Illinois,[2] and its 1831[3] extension is the principal meridian for land surveys in Wisconsin and northeastern Minnesota.[1][2] 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,[citation needed] runs west from this initial point.[1][2] The meridian and this baseline governs surveys in Illinois that are west of both the Illinois River and the third principal meridian.[2]
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°W and notes that the meridian is an extension of the line north from the mouth of the Illinois River near Grafton, Illinois.[1]
The meridian was extended north in 1831, through Wisconsin and northeastern Minnesota.[3] The extension uses the Illinois–Wisconsin border as its baseline,[1][2] and is the basis of surveys in all of Wisconsin, as well as that part of Minnesota:
The initial point of the extended fourth principal meridian is located at 42°30′27″N 90°25′37″W / 42.50750°N 90.42694°W.
