Fourth principal meridian

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's baseline, sometimes called the Beardstown baseline,[citation needed] runs west from this initial point.

[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 / 42.50750; -90.42694.