7 Professional Land Surveyors of Oregon | www.plso.org continues Featured Article EARLY AMERICAN SURVEYING EQUIPMENT By Dr. Richard L. Elgin, PS, PE Much of America’s surveying practice descended from the English, but our early surveying equipment did not. The Old World used the delicate, expensive theodolite to divide its lands, sighting on points and measuring angles on a divided, graduated circle. American surveyors needed to establish boundaries over vast wildernesses which were difficult to traverse and they needed to do it quickly and cheaply. Enter American innovation, technology and craftsmanship to improve a device used by mariners for hundreds of years, a form of which was being made in England, the magnetic compass. The result was the rugged, inexpensive standard American compass. As one commentator said of the American compass “where accuracy can be sacrificed to speed and cheapness.” The Compass The compass with its rugged body of wood or brass, two sight vanes, a leveling device and placed on a staff or tripod, required only a balanced magnetized needle resting on a sharp point. The needle aligned itself with the earth’s magnetic field and pointed to magnetic north. Magnetic north was known to move and hence was a poor direction with which to reference boundaries. This movement was well known, being noted in some 1746 instructions that it “…may in time occasion much confusion in the Bounds…and, Contention.” Variation, the angle between the the true meridian (a line of longitude) and magnetic north was known to differ at different locations on earth and the angle was known to change in amount over time and location. True north was a better reference direction and in 1779 Thomas Jefferson wrote that the plats of surveys were to be drawn “protracted by the true meridian” and the variation noted. The first standard American compasses were “Plain” compasses. They used magnetic north and had no mechanism for applying the variation angle, converting magnetic direction to true direction. David Rittenhouse (1732–1796) was an American man of science. He is generally credited with adding a vernier to the plain compass so one could “set off” the variation, the needle still pointing to magnetic north, but the bearing to the object sighted read on the compass circle being the true bearing. Thus the “plain compass” became the “vernier compass,” a great advancement in the American compass. The Land Ordinance of 1785 specified that all lines be surveyed “by the true meridian…the variation at the time of A rare solar compass by a very rare maker, John S. Hougham; Franklin, Indiana. Compass was made about 1861. All images are photos of instruments in Richard Elgin's collection.
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