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Dorchester County plot (Click to enlarge)

Charles County Manuscript (Click to enlarge)

The draftsman, in turn, reduced the measurements to paper (see example at left) [sjm04-63014-os1], laboriously plotting each course and distance at the end of the last, until the measurements were exhausted (and perhaps the draftsman as well!). Later, all the strings of lines were compiled into a single tracing (see Charles County manuscript at right [sjm 04-63033-os1]). Subsequently, the manuscript was transmitted to the lithographer for engraving and printing. Afterwards, each map was hand colored.

Equipment Problems
But all was not a bed of roses. William McCrerry, one of the surveyors involved in the preparation of the Harford County Map wrote in an 1865 letter to Mr. Martenet, "The odometer does not work so well as I should like. It is much worn, has too much play in the axis I think. Taking the wheel at 8 feet 8 inches the notes do not compare closely with the mile stones. I have been very careful in the bearings and distances and do not think that I could have made a mistake in the reading of the odometer. . . [l]et me hear from you at once, and say what you think of my work and whether you think the Odometer is working as well as you desire. If you could get another one and send me perhaps it would be as well to send this one to the maker and have it repaired. The wheel I find by measurement just made is 8 feet 10 inches in circumference."

McCrerry letter (Click to enlarge)

That difference in circumference amounts to just under 2 percent; if McCrerry was correct, the measured distances would be 2 percent smaller than the actual ground distances, unacceptable even for the time period.

Out of Control
From a modern surveyor's perspective, there was no formal control structure to the effort. In other words, all accurate surveys are based on a series of control measurements that (usually) embrace the perimeter of the project. These measurements tend to be the most precise measurements in the whole survey, and they form the geometric framework for the rest of the work. They are analyzed rigorously for mathematical integrity (making sure, for instance, that the endpoint of the survey coincides with the beginning point of the survey, etc.), and any errors detected are corrected prior to continuing. This results in a rigid structure for the survey, preventing warping and shifting of the junior measurements (easily seen in early mapping where small clumps of details are accurate within themselves, but not in the correct position with respect to other clumps of details). Proper control prevents those sorts of errors.

For large areas, a technique called "triangulation" was used for control. This involved measuring the angles of giant triangles, the verticies of which were on high points (or specially constructed towers) which could be seen from long distances (miles). Combining triangles together by using common sides on adjacent triangles allowed networks of these figures to be extended over vast distances. And measuring the lengths of only one side of the initial triangle and of the ending triangle would give a mathematician all they would need to trigonometrically compute the lengths of the sides of the intermediate triangles—and thus the relative position of those verticies—saving the cost and difficulty of measuring them directly. The Coast and Geodetic Survey used this method to extend networks of triangles completely across the continent. Those verticies were then the basis for subsequent junior surveys which took off from one and checked into another, further densifying the control. GPS, of course, turns all that on its ear, and has revolutionized the practice of control surveying. But that's fodder for a different century.



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Copyright © 2009 S.J. Martenet & Co., Inc.