The Method

Mason and Dixon set up observatories every five miles, taking a number of different observations on a number of different stars, and computing the precise latitude and longitude of each observation, used the mean of all of them to determine precisely the location of each observatory. With these scientific data they then marked the boundary line in accordance with the Court decisions and agreements between the Proprietors. Their line ran due West across the Susquehanna River and the Alleghany mountains to an Indian trail leading to Fort Duquesne. At this point the Indians refused to permit the line to be run further West. Mason and Dixon stopped at the Indian Trail and the final twenty miles of the Westward line was run later by other surveyors.

April, 1765. On the fifth on the month Mason and Dixon proceeded to run the West Line, using for the first 10-minute arc of the great circle a direction determined by calculations of spherical trigonometry and observations referred to above. Their measurements were from a “Post marked West” in Mr. Byran’s field, which in latitude was 15 miles south of the southernmost point in Philadelphia, and later turned out to be 2 miles 79 chains 27 links east of the northeast corner of Maryland. The survey proceeded westward, crossing the Little Christiana Creek, Great Christiana Creek and the Elk River. On the thirteenth a point had been reached near the end of the 10-minute arc of great circle, i.e., at 12 miles 25 chains from the point of beginning. At this time the scientists returned to the end of the line and came back with the astronomical transit or sector. The following day they set it up at the point reached on the thirteenth, to obtain a check on its latitude. The mean of the observations showed that the position of the sector was 1.29 seconds of arc north of the parallel through the Post marked West. A table of offsets was now prepared which took into consideration the distance of the great circle southward to the parallel and also the amount (129 feet) by with their line had erred from the desired great circle.

The calculated offsets were measured off from the great circle by returning eastward and a temporary monument was placed at every mile point. The positions marked the true boundary between Maryland and Pennsylvania. On the twenty-ninth of the month they proceeded to repeat the procedure, i.e., to run a second arc of great circle of 10-minute length. The following day they crossed the main branch of the North East River at a distance of 14 miles 2 chains from the Post marked West. At this time communications were dispatched to the commissioners from Maryland and Pennsylvania to inform them that the survey would reach the Susquehanna River in twelve days.

May, 1765. The line was continued without interruption for nearly two weeks, during which time the route crossed the Octoraro River three times in quick succession. The river was very sinuous but each crossing was nearly perpendicular to its banks. The three crossings respectively began at 20 miles 61 chains, 20 miles 71 chains, and 21 miles 25 chains, and the width of the river was recorded by Mason as about 50 yards. Conowingo Creek was crossed at 23 miles 67 chains, and at a distance of 25 miles 75 chains 57 links the end of the 10 minute arc of great circle was approximately reached…

From The Journal of Charles Mason and Jeremiah Dixon