Third principal meridian

The Third Principal Meridian extends northward from the Ohio River mouth to the northern boundary of Illinois, marking 89 degrees 10 minutes west longitude.

Established in 1815, this meridian point became the foundation for surveying 60 percent of Illinois land, including the Chicago metropolitan region.

The precise location of the original cornerstone demonstrates the scientific accuracy of early American land surveying methods that shaped settlement patterns.

Currently marked by a steel plate beneath Highway 51, the meridian point serves as a reference for modern land surveys and property boundaries.

This meridian forms part of the Public Land Survey System, dividing townships into eastern and western sections while intersecting with its baseline at 38 degrees north.

GPS coordinates: 38.47390,-89.14440

Latest update: March 10, 2025 12:00