The Third Principal Meridian begins at the mouth of the Ohio River

Ohio River

The Ohio River is the largest tributary, by volume, of the Mississippi River. At the confluence, the Ohio is even bigger than the Mississippi and, thus, is hydrologically the main stream of the whole river system, including the Allegheny River further upstream...

 and extends north to the northern boundary of the state of Illinois

Illinois

Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

, and with the base line in latitude 38° 28' 20", governs the surveys in the state east of the third principal meridian, with the exception of those projected from the Second Principal Meridian

Second Principal Meridian

The Second Principal Meridian coincides with 86° 28' of longitude west from Greenwich, starts from a point two and one half miles west of the confluence of the Little Blue and Ohio rivers, runs north to the northern boundary of Indiana, and, with the base line in latitude 38° 28' 20", governs the...

, and the surveys on the west, to the Illinois River

Illinois River

The Illinois River is a principal tributary of the Mississippi River, approximately long, in the State of Illinois. The river drains a large section of central Illinois, with a drainage basin of . This river was important among Native Americans and early French traders as the principal water route...

. This meridian is nearly coincident with 89° 10' 15" of west longitude from Greenwich.

External links

• Illinois State Historical Society: Historic Marker