Surveying the Mason-Dixon Line

The Mason-Dixon Line, the famous boundary between Pennsylvania and Maryland that came to symbolize the divide between North and South, was actually surveyed to settle a property dispute between the Penn and Calvert families over the exact location of their colonial land grants. English astronomers Charles Mason and Jeremiah Dixon spent four years from 1763 to 1767 surveying the line using the most precise instruments available at the time, placing stone markers every five miles along the border. The survey was considered a masterpiece of 18th-century precision and their names became permanently attached to the line.

Source: Wikipedia