In Illinois, for example, the third principal meridian — which roughly cuts it in two — is located about 9 miles west of the 89th geographic meridian. It was established as a line running true north from the point of confluence of the Ohio and Mississippi Rivers. Its exact location is 89 degrees, 10 minutes, and 30 seconds west of Greenwich, England. Greenwich is the starting point for all longitudinal measurements since longitude 0 degrees passes through it.
The fourth principal meridian was established for surveying lands located between the Illinois and Mississippi Rivers. It begins at a point near Beardstown and is along a line straight north from the mouth of the Illinois River near Grafton. The longitudinal reading of this line is 90 degrees, 28 minutes, and 45 seconds west.
The second principal meridian — used in descriptions of some land in Illinois — is located in Indiana. Its reading is 86 degrees, 28 minutes, and 0 seconds west.
Not all principal meridians are numbered. Some, such as the Michigan and Louisiana Meridians, are named for the state in which they are located. Others are named for territorial features; for example, the Indian Meridian in Oklahoma and the Salt Lake Meridian in Utah.
There are over thirty principal meridians in the United States and they do not always extend from one state into another. The third principal meridian is located entirely within Illinois. The fourth principal meridian, however, extends into and is the reference line for all land descriptions in Wisconsin — in addition to some in northeast Minnesota.
