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Most people think of the "Mason-Dixon Line" as being a term used during the Civil War to describe the boundary between slave-holding territory on the south and free territory on the north. And that is, indeed, what the term came to mean. But the Mason-Dixon Line itself was drawn almost a hundred years before the Civil War began, and it extended only 245 miles, which is hardly long enough to separate all the states in the North from those in the South.

The Mason-Dixon Line was, however, long enough to separate Pennsylvania from Maryland, and that is all that it was designed to do. The original charters for these two states were so loosely drawn that the Penn family laid claim to a sizable portion of Maryland, and the Calvert family in Maryland claimed land in Pennsylvania that included the city of Philadelphia.In 1760, the families finally agreed to have two English astronomers and surveyors - Charles Mason and Jeremiah Dixon - settle the dispute by establishing a boundary along a precise line of latitude, namely 39 degrees, 43 minutes, 17.6 seconds north latitude. So, in 1763, Mason and Dixon began their survey, and on October 18, 1767, they finished.

They placed stone markers at one-mile intervals all along the route, each bearing the crest of the Penn family on one side and the crest of the Calvert family on the other. Many of these markers were removed by colonial souvenir hunters, but most have now been recovered and have been replaced (securely) in their original positions.

So accurate was the original survey conducted by Mason and Dixon that when the line was resurveyed in 1849, and again in 1901, no error of significance was found.

I find that absolutely astounding. I mean, just think about the difficulty of drawing a straight line that runs due east and west, for hundreds of miles, over hills and valleys, through forests and swamps, along a very specific line of latitude. Now think about performing this task with the knowledge and equipment and in-stru-ments that were available in the 1760s.

Today, you and your children can determine your exact position on Earth - longitude, latitude and elevation - very easily by taking advantage of the extremely precise surveys that have been conducted throughout the United States by various government agencies. You can find several survey markers within a short distance from your home that can tell you these important details, if you know where to look for them.

These markers, which are called "bench marks," are bronze discs (about 4 inches in diameter) that have been set in concrete and bear an inscription naming the agency that placed them, along with an identifying number. You can find out where these bench marks have been placed in your area by looking at a "quadrangle map" (or "7.5 minute map") published by the U.S. Geological Survey. Such a map may be part of your public library's collection, or you can obtain one of your own from the USGS for just a few dollars.

Once you locate a bench mark, you can just call the agency named on the disc (USGS at 303-236-5812 or U.S. Coast and Geodetic Survey at 301-443-8631, for example), give them the identifying number, and they will describe the position of that marker with a precision of at least 2, and sometimes 3 or 4, decimal places!

For example, I called the USGS about the disc that is embedded in the sidewalk outside the county courthouse near where I live, and I was told that that disc is precisely 719.490 feet above mean sea level, and its position on Earth is precisely 41 degrees, 53 minutes, 9.38 seconds north latitude; and 88 degrees, 18 minutes, 30.20 seconds west longitude.