The surveys of the Third principal meridian were finished in 1815. It runs parallel to Indiana's Second principal meridian and Ohio's First Principal Meridian. Approximately sixty percent of the state of Illinois was surveyed from the Third Principal Meridian. The principal base line of the Second Principal Meridian, or the Centralia base line, was extended westward to the Mississippi River to intersect the Third principal meridian at an initial point in the southern part of Illinois. The marker of this initial point is now under the surface of Highway 51. Survey townships run north to the Indiana-Michigan border, and south to the Ohio River. Ranges are numbered to both the east and the west of the Third principal meridian terminating at the Illinois River in the west and the Illinois-Indiana border in the east.
