Meridians are imaginary lines of longitude on the earth that extend from the North to South Pole. A Principal Meridian is one which is used as a reference line to survey a large area.
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.
Principal meridians 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 Wisconsin and is the reference line for all land descriptions in Wisconsin and for some in northeast Minnesota.
Base lines are parallels of latitude used as the north-south reference of such surveys.
The true east-west line under the rectangular survey system is called a base line. Like the principal meridians, the base lines were arbitrarily established by the government surveyors.
There is at least one base line for each meridian. The base line for the Second and Third Principal Meridians in Illinois is the same. It is a latitudinal line with a reading of 38 degrees, 28 minutes, and 20 seconds north of the equator. It intersects the Third Principal Meridian at a point near Centralia, Illinois.
The base line for the Fourth Principal Meridian runs straight west from the beginning of that Meridian near Beardstown. The geographical location of this line is 40 degrees, 0 minutes, and 30 seconds north. The Fourth Principal Meridian has a second base line used for describing land in Wisconsin and parts of Minnesota. The base line coincides with the Illinois-Wisconsin border.