At night, Charles Mason and Jeremiah Dixon studied the movement of stars and made endless, complex calculations. Their clever combination of astronomy and surveying resulted in the Mason-Dixon Line, a 325-mile border through the wilderness of West Virginia, Pennsylvania, Maryland and Delaware.

Starting in 1763, the intrepid Englishmen scaled mountains and risked their lives on a frontier where Colonists and Indians butchered each other in the French and Indian War's aftermath. For most of this arduous odyssey, their prized equipment, a telescope called a zenith sector, traveled on a feather mattress in a two-wheel chariot.

Accompanied by ax men who hacked a path through thick forests, the precise, professional duo placed their last boundary stone atop Brown's Hill in West Virginia, 242 years ago today. (Indian guides halted Mason and Dixon on Oct. 9, 1767. Surveyor Andrew Ellicott finished marking the last 22 miles of the Mason-Dixon Line in 1784.)

Nearly 21/2 centuries later, many original stones have disappeared or suffered damage. Using sophisticated global positioning equipment, 100 professional surveyors have volunteered to document and preserve the stones along the 233-mile border that marks Pennsylvania, Maryland and West Virginia. Their goal is to finish by November 2013, the 250th anniversary of Mason and Dixon's arrival in Philadelphia.

"We've located 200 of the stones," said Todd Babcock of Fleetwood, Berks County, a professional surveyor who has worked on the project for the past 18 years.

But, before heading to Western Pennsylvania, Mason and Dixon marked the 92-mile, north/south boundary between Delaware and Maryland.

That was because William Penn and his family, who owned Pennsylvania, also controlled the land that became Delaware.

After establishing the southernmost point of Philadelphia, the surveyors headed south to Chester County. Then, they measured 15 miles south to what they called the "Post Mark'd West," the reference point for the entire survey. The original post was lost; to replace it, a granite stone was installed in the 1950s in Delaware's White Clay Creek State Park.

Preservation efforts progressed as technology improved. When the Mason-Dixon Line Preservation Partnership began in the early 1990s, pictures were taken in black and white. Then the Internet boomed, color digital photography became standard and by the mid-1990s, global positioning equipment was available.

"That's when we started locating the stones. We'd go out with 20 to 40 people and 15 receivers trying to locate the things," Mr. Babcock said. Until then, they relied on U.S. Geological Survey maps that were not detailed.

To mark Pennsylvania's southern border, Mason and Dixon began work in Philadelphia outside a house that no longer stands.

"They had to establish the southernmost point of Philadelphia as it existed at that time," Mr. Babcock said. "It's on South Street. I went down and surveyed that intersection. We worked backward to determine where that house was. The boundary of Pennsylvania was placed 15 miles south of South Street in Philadelphia," he added.

For the next 132 miles, Mason and Dixon used stones carried by boat from England to mark the border. Then, they reached Sideling Hill, a spot that straddles Pennsylvania and Maryland.

"The mountains were too steep. They couldn't carry the stones over the mountains," Mr. Babcock said. So, wooden posts were stuck in the ground and encircled by piles of stones or earth.

The Mason-Dixon Line settled a property dispute between the Penn family, which owned Pennsylvania and Delaware, and the Calvert family, which controlled Maryland. The two families, fought an 80-year legal battle over the land before hiring Mason and Dixon.

Edwin Danson, a British surveyor who lives outside of London, is the author of "Drawing the Line: How Mason and Dixon Surveyed the Most Famous Boundary in America." The Penn and Calvert families, Mr. Danson said, spent somewhere between $2 million and $3.2 million for the survey, including the surveyors' labor, a large crew, plus the cost of making and importing boundary stones from England.

"It would probably cost about the same today using modern equipment," the author added.

Mason and Dixon, Danson said, each earned about $145,000, or about $36,250 a year, which he called paltry but noted that it was tax free.

Mason, a widower who remarried after he returned to England, loved America and moved his family to Philadelphia in 1786 but died there later that year. Dixon, who never married, returned to County Durham and his native Cockfield. He died in 1779 at age 45.

In the early 1800s, many people used the Mason-Dixon Line as a synonym for the division between Northern and Southern states.

G. Terry Madonna, Franklin and Marshall Scholar at the Center for Politics and Public Affairs, said the boundary, "was wrongly used, throughout much of American history ... to describe those states that were free and those states that held slaves."

"Delaware had slaves but did not leave the Union. Maryland had slaves and didn't leave the Union, but a good portion of its men volunteered to go fight for the Confederacy," Dr. Madonna added.

When he taught Pennsylvania history, Dr. Madonna used North and South to describe the Union and the Confederacy and avoided references to the Mason-Dixon Line, fearing that it may "leave students with the impression that it was a firm dividing line."

Pennsylvania passed a gradual emancipation law in 1780, but that year the state had 6,000 slaves. The law was so restrictive that, "We had slavery in Pennsylvania into the 1830s and 1840s," Dr. Madonna said.

To pundits, the Mason-Dixon Line is more a political than a geographical boundary. During last year's presidential election, commentators used it as political shorthand to describe key voting blocs -- conservative Reagan Democrats, suburban Republicans and black Democrats all wield power in the South.

"Every state legislature north of the Mason-Dixon Line in the Northeast is in Democratic hands, with the exception of the state Senate in Pennsylvania," Dr. Madonna said.

Mr. Babcock, 45, has traveled the entire Mason-Dixon Line and will talk this month with filmmakers creating a documentary about it for the History Channel.

A genial father of two, Mr. Babcock is eager to see a wayside exhibit built near 30 South St. in Philadelphia, where his heroes began their work. His organization has raised $10,000 toward that effort.

His fascination with the boundary was inspired by Charlie Bitler Jr., a retired surveyor who lives in Pottstown, Montgomery County, and began photographing stones in the 1970s after he realized many were damaged. Mr. Bitler and other surveyors gathered in Emmitsburg, Md., and decided to do an inventory.

The task required crossing farmers' fields, knocking on doors and talking with strangers. Some stones have been repaired with new tops; others were replaced with new granite stones.

"Pennsylvania has more of a network of secondary roads that you can drive around to get to the stones," Mr. Babcock said.

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