Third principal meridian

The Third Principal Meridian runs northward from the Ohio River mouth to Illinois's northern border and marks 89 degrees 10 minutes west longitude. A steel plate beneath Highway 51 shows the exact survey point today.

This survey point was established in 1815 and became the foundation for mapping roughly 60 percent of Illinois land. The resulting surveys enabled systematic land distribution and settlement across the region.

The precise location demonstrates the scientific care of early American land surveying that shaped how settlements grew. This accuracy allowed settlers to clearly define property boundaries and avoid disputes.

The point sits beneath a busy road so direct access is limited, but its exact location is easy for surveyors and property owners to find. Visitors should know this historical reference point is mainly relevant for professionals working with land records.

This meridian forms part of a larger system for surveying land across the US and divides sections into eastern and western parts. It intersects with its baseline at 38 degrees north and creates a grid that property claims still rely on today.