Third principal meridian

The Third Principal Meridian is a north-south line of longitude that serves as a reference point for land surveying in the United States, specifically in the region west of the Ohio River. Established by the U.S. government in the early 19th century, it is one of several principal meridians used in the Public Land Survey System (PLSS). The Third Principal Meridian runs through an area that includes parts of the states of Illinois, Missouri, and Wisconsin.