Third Principal Meridian

The establishment of the Third Principal Meridian followed Indiana’s Second Principal Meridian and Ohio’s First Principal Meridian. It was surveyed in 1815, to measure the land that would soon become the State of Illinois. The meridian originates from a Point of Beginning at the intersection of a baseline in the southern part of the state. The original marker is now under the surface of Highway 51. A monument was installed next to the road in 1976 to commemorate the Initial Point, but also to serve as an officially surveyed offset point (known as a witness corner), since the original location is in the middle of a highway now.

2327 Sixty percent of Illinois (in red) was surveyed from the Third Principal Meridian.

2874 CLUI photo.
The Initial Point was recently dug up and made more accessible with the installation of a manhole cover.