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History of the Mason-Dixon Survey

Between 1763 and 1767, surveyors Charles Mason and Jeremiah Dixon established what would become known as the Mason-Dixon Line. The boundary was surveyed and drawn in order to resolve a border dispute between England and the American colonies, and it established various parts of the borders of Pennsylvania, Delaware, Maryland, and what is now West Virginia. Colloquially, the Mason-Dixon line is thought of as the geographic and cultural border between the northern and southern United States.

The land between the 39th and 40th parallels was the subject of a dispute between the colonies of Pennsylvania and Maryland. Both colonies laid claim to the land, and the dispute eventually erupted into a series of violent incidents known as Cresap’s War. Charles Mason and Jeremiah Dixon were ordered to survey the borders between Pennsylvania and Mayland as part of the conflict’s resolution. Mason and Dixon also determined the boundary between Delaware and Pennsylvania, a section of the border between Delaware and Maryland, and portions of western Virginia, which would break off into West Virginia during the Civil War.

In April 1765, Mason and Dixon began surveying the Pennsylvania-Maryland border, which would eventually become the most famous aspect of their work. They were commissioned to establish the border five degrees of longitude west of the Delaware River, creating Pennsylvania’s western border.

To mark the lines they drew, Mason and Dixon placed stones on the ground at one-mile intervals. The stones had “P” engraved on the Pennsylvania/Delaware side of the border and “M” engraved on the Maryland side. Some of the original stones remain intact to this day (with added protection from metal cages). While doing this, Mason and Dixon also verified their previous work, which delineated Delaware's southern boundary from the Atlantic Ocean to the "Middle Point" stone (along what is today known as the Transpeninsular Line). They proceeded nearly due north from this to the Pennsylvania border.

Mason and Dixon were stopped short of completing their goal after encountering a Native American tribe that halted their progress. At this point, they were about 250 miles west of the Delaware River. In 1784, surveyors Andrew Ellicott and David Rittenhouse finished the work Mason and Dixon had started.

As technology has advanced and surveying equipment has improved, Mason and Dixon’s work has been refined and adjusted. However, their work was incredibly accurate, requiring only minor modifications. Some of Mason and Dixon’s stones were placed up to a few hundred feet away from their intended locations, but their margin of error was so small that the lines they drew still constitute the legal boundary between Pennsylvania and Maryland.

The Pennsylvania-Maryland border was resurveyed in 1849 and again in 1900, but Mason and Dixon’s work had stood the test of time in its quality and its historical relevance. The term “Mason-Dixon Line” came into common usage well after the surveyors’ deaths, when the line was used to divide free states and slave states as part of the Missouri Compromise. Although Maryland is no longer considered a southern state by most Americans, the Mason-Dixon line still represents the geographical divider between northern and southern culture. Charles Mason continued to work in astronomy until his death in 1786, and was a close collaborator of Benjamin Franklin. Jeremiah Mason set sail to explore England and Norway until his death in 1779.

Modern-day surveyors can find in plenty of inspiration in the work and legacy of Mason and Dixon. Although they lacked modern surveying equipment like marking paint, GPS location, or 3D scanners, the two men did excellent work by the standards of the time and cemented their places in American history.

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