(WHTM) — 260 years ago today, Charles Mason and Jeremiah Dixon arrived in Philadelphia to begin their survey of the boundary lines between Pennsylvania and Maryland.
It would take them almost five years to complete the work.
Artist’s conception of Mason and Dixon conducting their survey. (Credit: Library of Congress) 
The New Castle Courthouse in Delaware is one of the locations from which measurements for the boundary lines were taken. 
This small kiosk along Route 54 in the Delmarva Peninsula houses the marker stones for the southwest corner of Delaware. It is from here Mason and Dixon started surveying the tangent line. 
The stones in the kiosk. The stones on the left are from surveys done before the Mason-Dixon; the one on the right is a M-D survey “Crown Stone” since it has the Calvert crest on one side, the Penn crest on the opposite side, and crowns over both crests, to remind everyone who has the final word here. This, in fact, is a “Two-Crown” stone; because it’s at a corner, it has two of each crest. 
An early map of the Mason-Dixon Line (Credit: Library of Congress) 
A replica “Crown Stone” at Mile 40 on the West Line. Located at the intersection of Norrisville Road, Harford County, Maryland, and Marsteller Road, York County, Pennsylvania. 
The replica “Crown Stone” on the right, what’s left of the original Crown Stone on the left. Times have been hard on some of the markers. 
A replica mile marker. These were set at every mile along the survey, and had “M” on one side and “P” on the other. 
This particular replica stone, along Route 45 south of Shrewsbury, is filling in for a stone in a cornfield about 400 yards away.
The actual survey work couldn’t start until spring of 1764. Mason and Dixon spent much of the winter figuring out where to start.
The east-west boundary line was to be measured along a latitude fifteen miles south of the southernmost point of Philadelphia. Mason and Dixon had to work out the precise latitude, which involved a lot of math and a lot of measurements of the movement of stars.
Most of those were done while peering through astronomical instruments while lying on their backs, in the dead of winter, in temperatures well below freezing. Nobody said being a surveyor was easy.
Having established the starting point for the “West Line,” Mason and Dixon spent 1764 surveying a completely different boundary. This was the Tangent Line: the north-south boundary between Maryland and Delaware, which was, at the time, part of Pennsylvania.
Both mathematically and geographically, this was the toughest part of the survey, and one can’t help wondering if they tackled it first because they wanted to get it out of the way.
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Mathematically it was difficult because they were measuring a diagonal, or tangent. This meant instead of simply tracking along a line of longitude, they had to calculate everything using trigonometry. (Then they checked their measurements with more star observations.)
As for geographically, they had to cross rivers and creeks, slog through wetlands, and chop their way through forests. (A crew of axmen cleared an 8- to 9-foot-wide path so Mason and Dixon could conduct their measurement.)
In 1765, they started on the West Line. It would take them until Oct. 9, 1767, to complete their survey — 233 miles later.
But they still had one more task to complete: creating about 200 copies of a map and plan of the surveyed line. Even with the help of the printing press, this took time. So it was that Charles Mason and Jeremiah Dixon finally boarded a ship to return to England on Sept. 11, 1768, almost five years after they first set foot in Pennsylvania.
