Illinois' Third Principal Meridian: Location and Significance

The Third Principal Meridian is a north-south survey line governing land descriptions in central Illinois.

Geodetic Position

• Longitude: 90° 28′ 55″ West of the Prime Meridian (Greenwich)
• Approximately 89° 55′ W in modern NAD83/WGS84 (datum shift)
• Runs from the Wisconsin border (north) to the Ohio River (south)

Survey Role

Established 1805 by U.S. Surveyor General Jared Mansfield. Serves as the reference meridian for the Third Principal Meridian Survey, covering central Illinois (roughly 89°–91°W). Townships are numbered north/south from the Centralia baseline; ranges east/west from this meridian.

Modern Relevance

• Legal descriptions (e.g., "T2N R3E 3rd PM") still reference it
• GIS layers, county parcel maps, and title work depend on original monumentation
• Some original bearing trees and stone monuments survive; others restored by GPS

Illinois has two principal meridians: 2nd PM (90° 12′ W, governing western IL) and 3rd PM (governing central/eastern IL). The 4th PM (90° W) governs Wisconsin/Minnesota.