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The March 2014 issue of PE (Professional Engineer) Magazine has an interesting article on the 1763 - 1768 survey of the Mason-Dixon line between Pennsylvania and Maryland. Although the line was later known for other reasons, it was originally surveyed to settle a dispute between the Calverts, who were granted Maryland as the land below the 40th parallel and the Penns, who were granted Pennsylvania as the land north of the 40th.
After several local tries, Mason and Dixon were brought, at the suggestion of the Royal Astronomer, in from England to make the definitive survey. For various reasons stated in the article, the line ended up south of 40 degrees north.

Of interest is the use of navigational instruments - a Hadley Quadrant, an Octant, and non-navigational Zenith Sector, transit, equal altitude instrument, and a pendulum clock. The line of latitude was established by star sights "and complex astronomical observations".

A lot of emphasis is on measuring south exactly 15 miles from the first latitude reference of 39-56-20. The final latitude is not stated but, from other sources, I found that it was 39-43-26.4 which works out closely using 6076 feet/nm and /minute of latitude and 5280 feet/sm.
A 16' wide by 330 mile long clearing was made with axes. One of the remarks from other sources was that, from a hilltop, the surveyors could see the slight curve of the line as it followed the parallel.
From other documents I read that later checks of this early survey find little or no error.

Many of the stone markers placed in the 1760's are still in place.

PE Magazine kindly provided a link:
http://www.nspe.org/resources/pe-magazine/march-2014/epic-survey-mason-and-dixon

Regards, Noell

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