When Charles Mason and Jeremiah Dixon arrived in Philadelphia 250 years ago, the city extended from just below South Street to Vine Street in the north and to about Seventh Street in the west; with a population of about 22,000, it was the largest city in colonial America.

The two English surveyors promptly began working on what four years later would become one of the foremost scientific achievements of the 18th century: using the stars to accurately measure more than 300 miles of land.

Their efforts would settle a bloody, three-generation border dispute between the proprietary families of Pennsylvania and Maryland, but when Mason and Dixon left Philadelphia in 1767, their work was soon forgotten. Indeed, the official report of the survey didn't even mention their names, and almost certainly neither man ever heard or saw the phrase Mason-Dixon Line.

But a half-century later, as the national debate over slavery heated up, the line would take on a metaphoric meaning - the border between freedom and slavery - and the Mason-Dixon Line became one of the world's most famous boundaries.

This symbolic fame has overshadowed the fact that at the time the line was the greatest geodetic survey ever undertaken and set a new standard for surveying and mapping. This month, a group of Mason's and Dixon's professional descendants hopes to rectify this oversight.

The Surveyors Historical Society is holding three days of events, all open to the public, in and around Philadelphia to celebrate the line's sestercentennial. From Aug. 29-31, The "Surveyors Rendezvous 2013" will bring together a large contingent of land surveyors, history buffs, and antique-instrument experts.

It all began 250 years ago at what today is a pedestrian bridge extending South Street over I-95. In 1763, it was the site of a house at the southernmost point in the city.

The exact location of the house was only recently confirmed by researchers paging through centuries-old property records. This research resulted in the designation of the site near Front and South Streets as a national historical site, and a marker will be placed there Aug. 30.

While the line didn't start here, it was used as a reference point for all future measurements. From Philadelphia, Mason and Dixon traveled 31 miles due west to Embreeville in Chester County to put themselves directly above the Delmarva Peninsula. Here they placed a stone that they used to determine latitude and longitude from the stars. It became known as the "Stargazer's Stone," and today it is an icon in American surveying history. A historic marker and plaque will be placed here Aug. 30.

Mason returned to Philadelphia in 1786, died soon thereafter, and is buried in an unmarked grave in Christ Church Cemetery, along with Benjamin Franklin, whom he had befriended while working on the Line. An original line limestone marker will be placed at the site of Mason's grave in ceremonies Aug. 31.

Also in conjunction with the 250th anniversary, the surveyors have completed an inventory, begun in 1990, to locate all 230 limestone markers that were set at one-mile intervals by Mason and Dixon. Over time, these markers had been damaged by weather, age, and vandalism, yet survived through the Revolutionary and Civil Wars, the Industrial Revolution, and 20th-century expansion.

Using satellite technology, an organization called the Mason and Dixon Line Preservation Partnership has located nearly all of the stones, and it purchased granite markers to replace a few missing ones. Group members marvel at how accurate Mason and Dixon were. Todd Babcock, the leader of the group, said: "What they were able to do is simply amazing, especially considering the instruments that they were using and the conditions they were battling. To be able to mark a continuous line of latitude this precisely gives testament to their brilliance."

Babcock, a professional surveyor, has spent more than two decades leading the effort to locate, catalog, and restore the stones, which were quarried in England. "With all of our modern technology, modes of communication, and ease of transportation, this would still be a daunting task to complete today," he said. "The logistics of the undertaking of this project are incredible. I feel a kinship with Mason and Dixon and every time I go out on the line I learn something."

The line begins at the Atlantic Ocean at Fenwick Island, Del., extends westward about 35 miles to the halfway point between the Atlantic and the Chesapeake Bay. Here it turns north and runs about 87 miles to a point near Newark, Del. (Delaware was once part of Pennsylvania.) From here it goes west some 230 miles to a point between Pennsylvania and what is now West Virginia.

Just 13 years after Mason and Dixon left America, on March 1, 1780, Pennsylvania abolished slavery, and the line became the division not just between Pennsylvania and Maryland but between a free state and a slave state. By 1854, historian John H.B. Latrobe said: "There is perhaps no line, real or imaginary, on the surface of the earth, whose name has been oftener in men's mouths during the last 50 years. In the halls of legislation, in the courts of justice, in the assemblages of the people, it has been as familiar as a household word."

It remains one of the world's most unusual borders. Most boundaries between states and nations have been determined by tangible factors - rivers, the crests of mountains, or at least by even parallels of latitude and meridians of longitude. But because of the court-decreed requirement that it begin exactly 15 miles south of southernmost Philadelphia, the east-west portion of the line conforms to the quirky latitude of 39 degrees 43 minutes and 17.6 seconds north.

Although their last names would become household words, Mason and Dixon are almost unknown as people. No likeness of either has ever been found. It is known that they were a symbiotic pair - Dixon was a Quaker bachelor and a dandy who was thrown out of the religious society for drinking; Mason was a morose, teetotaling Anglican.

Yet from these unknowns came a line that would be embedded in the national psyche as a powerful racial symbol, one that surveyors will celebrate later this month as an important scientific achievement, drawn across the colonial wilderness while facing daily danger from blizzards, wild animals, and hostile Indians.

William Ecenbarger is a former Inquirer reporter. E-mail him at william.ecenbarger@gmail.com. For information about the Surveyors Rendezvous, visit www.surveyorsrendezvous.org.