A map of the proposed boundary between Maryland and Pennsylvania, circa 1750.

In 1768, astronomer Charles Mason from the Royal Observatory in Greenwich and surveyor Jeremiah Dixon completed what is arguably one of the greatest scientific and engineering achievements of all time-the survey of the Mason-Dixon Line. Commissioned to mark the disputed boundary between the Colonial provinces of Maryland and Pennsylvania, the line was the first professional geodetic survey completed in the world. Mason and Dixon spent almost five arduous years on the task, driving the line for 312 miles beginning in what is now Delaware on the eastern shore, then following a constant latitude west from a point 15 miles south of Philadelphia and ending at the “Great Warrior Trail” in the territory of the Six Nations. Later, the Mason-Dixon Line would gain mythic status as the boundary between North and South, between free and slave-owning states, somewhat overshadowing the extraordinary scientific achievement that it represented.

“How to determine longitude had just been discovered by John Harrison at about the same time as Mason and Dixon did this survey, and a degree of latitude had never been measured in the New World before,” says David Thaler, PE, LS, president of D. S. Thaler and Assoc. Inc., Baltimore, and a trustee of the Maryland Historical Society. “Their achievement was monumental.”

Many of the documents and artifacts from the survey still exist. For the first time, all of these items are on display in an exhibition at the Maryland Historical Society in Baltimore.

The final invoice for the survey work performed by Mason and Dixon.

Treasures from Yesterday

The exhibition, “Mason and Dixon and the Defining of America: Treasures from the Maryland Historical Society and Independence Hall and the Maryland State Archives,” features the 1763 parchment of the Articles of Agreement between Thomas and Richard Penn of Pennsylvania; Frederick Calvert, the sixth Lord Baltimore of Maryland; and Charles Mason and Jeremiah Dixon for surveying the boundary, including signatures and wax seals for all parties. It also houses the hand-written journal that Mason kept during the five-year task as well as the final invoice for the survey. “It’s in pounds, schillings and pence, but as near as I can figure in today’s dollars, the half of the bill that went to Lord Baltimore was about $900,000,” Thaler says. “In those days, 50 pounds a year was a very comfortable salary. Mason and Dixon were making 365 pounds a year so they were pretty well paid.”

Visitors can view field notes and calculations from the survey and letters written to William Penn and Lord Baltimore from Mason and Dixon along with field books from the Colonial surveyors who laid out the line incorrectly. “These were beautiful journals written in calligraphy, and you can see why they couldn’t complete the survey because you can find their errors,” Thaler says.

When Mason and Dixon completed the survey in 1768, 200 maps measuring 76 inches long by 27 inches wide were printed by Robert Kennedy of Philadelphia from Mason and Dixon’s hand-drawn copy. The boundary commissioners for both Colonies signed five or six copies of Mason and Dixon’s “true and exact” plan and affixed their wax seals. The Maryland Historical Society owns two copies of the signed and sealed maps, one of which is part of the exhibition.

Mason’s hand-written journal.

Besides the historic documents, Thaler has been able to recover the only surviving instrument used in the survey-the transit and equal altitude instrument that was commissioned by Thomas and Richard Penn from London’s master instrument maker John Bird especially for use by Mason and Dixon. The instrument, which enabled the professionals to determine true north by tracking stars where they crossed the meridian, was later housed in a small observatory behind Independence Hall in Philadelphia. It was on the steps of this observatory that the Declaration of Independence was first proclaimed to an assembled throng on July 8, 1776. After the Revolutionary War, the instrument disappeared. It was rediscovered in 1912 hidden under the floorboards of the observatory’s bell tower beneath the supports of the Liberty Bell. According to Thaler, the reason it was hidden is a mystery. “Whatever the reason,” he says, “the little transit on display at the Maryland Historical Society may be the most historic American scientific instrument ever, having  witnessed two of the seminal events in American history-the survey of the Mason-Dixon Line and the first reading of the Declaration of Independence.”

Other equipment used by Mason and Dixon to perform their survey included Gunter’s chains, compasses, theodolite transits, a zenith sector (a 6-foot telescope mounted on a 6-foot radius protractor scale used to determine latitude by measuring the angles of reference stars from the zenith in the sky), astronomical regulators and a Hadley’s reflecting quadrant (a measuring instrument used in navigation). While the Hadley’s quadrant was destroyed in a fire in the 19th century and the zenith sector has not been recovered, other early surveying tools-including Gunter’s chains, vernier compasses and a theodolite transit-are on display in the exhibition. Many of these pieces come from Thaler’s own collection.

“What’s fascinating to me is that this entire survey was done very much like any of us would have done it until the 1970s, when lasers and GPS technology started to become prevalent,” Thaler says. “They used the exact same techniques that I learned in college in the 1960s-300 years later. And the documents could have been written yesterday. That’s what I find so thrilling about all of it.”

The transit and equal altitude instrument used by Mason and Dixon in the survey.

A Personal Piece of History

The exhibition celebrates a significant geographic and symbolic dividing line in America’s past. For surveyors, it also represents a personal piece of history.

“Surveying is part of a very old and honorable tradition,” Thaler says. “When we do a survey, we’re told to follow in the footsteps of the prior surveyor. These [documents and artifacts] are the ultimate footsteps-the original footsteps of the guys who really started our profession here in America in Colonial days.”

Mason and Dixon and the Defining of America will be open to the public from Dec. 4, 2008 to Feb. 28, 2009. The exhibition is curated by Jeannine Disviscour, deputy director for collections and curator at the Maryland Historical Society.

For more information, call 410/685-3750 ext. 321, or visit www.mdhs.org.

Web Exclusive: Click hereto view David Thaler’s historical account of the Mason-Dixon survey in “National Treasure.”