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History buffs and land connoisseurs will enjoy David Thaler’s closing keynote presentation at the recent MSBA Land Use Institute held on October 5, 2021. In delightful staccato, Mr. Thaler explained the history of the geographical footprint that is today known as the state of Maryland.  Of course, the chronological history of the area of Maryland cannot be discussed without first talking about the eighty year legal boundary dispute between Maryland and Pennsylvania that Mr. Thaler describes as the “granddaddy of all title disputes.” 

The origins of the dispute arose because of the discrepancies between the grants awarded to George Calvert (Lord Baltimore) and William Penn by the English monarchy. On July 20, 1632, the land charter granted to Lord Baltimore was from the Atlantic Ocean “…unto the true meridian of the first fountain of the River Potowmack” and from the South Bank of the Potomac river to all land “which lieth under the Fortieth Degree of North latitude.”  (emphasis added) William Penn was granted the province of Pennsylvania and given all the land from 5 degrees longitude west of the Delaware River from the 42nd parallel of latitude down to the 40th degree latitude excluding a 12 mile circle around Newcastle in what is now Delaware. 

The maps of the time were based upon the survey conducted by Captain John Smith (Jamestown) of the Chesapeake Region in 1608.  The problem was that Captain John Smith’s maps inaccurately measured the 40th parallel too far south.  It was this mistake that set off one of the greatest boundary disputes in U.S. History between Maryland and Pennsylvania.  The dispute raged for more than 80 years with the big issue being “Where is the 40th parallel?”  You can listen to Mr. Thaler’s excellent chronological historical summary that led to this dispute below:

At stake was approximately 4,000 square miles and the city of Philadelphia.  The city of Philadelphia was actually about five miles south of the 40th parallel.  This would have placed Philadelphia in Maryland, not Pennsylvania.  It was unclear to whom taxes were due in this geographical area and violence broke out regularly along the border leading one of the colonists in Maryland to describe Philadelphia as, “…one of the prettiest towns in Maryland.”  The British Crown had to step in and order the Calverts and Penns to restore order.  In 1732, they agreed that the boundaries should be run 15 miles south of Philadelphia, but unrest on the border continued. 

In an effort to resolve the issue once and for all the Calvert and Penn families hired two surveyors recommended by the Astronomy Royal in England.  Charles Mason and Jeremiah Dixon were hired to survey the land using the most sophisticated tools of the day. The state of the art equipment included a Zenith Sector which was the most advanced instrument to determine latitude, an Octant (Hadley Quadrant) which measures angles up to 90 degrees and a Transit and Equal Altitude instrument used to determine true north by tracking stars where they cross the meridian.  The Calverts and Penns signed a written contract to pay Mason and Dixon time and materials plus expenses. It would take Mason and Dixon five years to accomplish this task after arriving in Philadelphia on November 15, 1763. 

Mason and Dixon were able to determine that the true Latitude of Philadelphia was 39 degrees, 56 minutes 29.1 seconds North.  The magnitude of Mason and Dixon’s accomplishments are profound.  They spent five years in America living in tents and enduring hot summers and frigid winters.  The Mason-Dixon line has been surveyed many times and the accuracy with which they achieved given the technology of the day continues to amaze. 

It seems that they played as hard as they worked.  One of their supply lists from 1764 includes 20 gallons of whiskey, 40 gallons of brandy and 80 gallons of wine.  Mason and Dixon were obviously taking leave of their all expenses paid contract.  In total, Mason and Dixon were paid 3,516 pounds, 9 shillings.  In today’s dollars that is approximately three hundred thousand dollars ($300K or $60K per year).  On  September 11, 1768, with their work complete Mason wrote, “thus ends my restless progress in America.”

In addition to being the first geodetic survey in America and one of the greatest scientific achievements of all time, the Mason-Dixon Line became an icon – the dividing line between slavery and freedom.  In 1820, Congress adopted the Missouri Compromise and first used the term “Mason-Dixon Line” to describe the Maryland Pennsylvania border.  States north of the Mason-Dixon Line, generally with a couple exceptions, were to be free and those south were slave states.  

The actual Diary and Field Journal of Mason and Dixon is in the Maryland State Archives.  Mr. Thaler found the “holy grail” of Maryland real property law when he stumbled upon an original signed and sealed map of the Mason-Dixon Survey.  The map is six (6) feet long and on parchment paper.  There are only 3 extant copies and two of those copies are located in Baltimore.  

Mr. Thaler, in true Indiana Jones fashion, found the Transit and Equal Altitude instrument used by Mason and Dixon to determine true north on the second floor of Independence Hall in Philadelphia and later contributed to the instrument’s restoration.  The Transit and Equal Altitude Instrument that Mr. Thaler re-discovered at Independence Hall was utilized or present for three of the seminal events in American History: 1) Mason and Dixon used the instrument in their historic survey; 2) the instrument was also used for the 1769 transit of Venus Observation; and 3) the instrument was observed in the background at the very first public proclamation of the Declaration of Independence to a crowd in Philadelphia.  It is for those reasons that Mr. Thaler believes the Transit and Equal Altitude Instrument that Mason and Dixon used in the survey is “arguably America’s most historic scientific instrument,” he said. 


David S. Thaler, PE, LS, DFE, D.WRE, EWRI, FRGS, F.ASCE, F. NSPE, is the principal owner of D.S. Thaler & Assoc., LLC in Baltimore, Maryland.  In addition to publishing five books of literary criticism, Mr. Thaler holds Maryland’s highest military order – The Distinguished Service Cross.  He is a board certified Diplomate of both The American Academy of Water Resources Engineers and the National Academy of Forensic Engineers.  In 2018, David won the PEPP award of the National Society of Professional Engineers as the Top Consulting Engineer in the U.S., and in 2019 he won the NSPE award as the Top Professional Engineer in the nation.  He is also the founding president of the Home Builders Association of Maryland Land Development Council, and was president of the Maryland Society of Professional Engineers.