Surveying Instruments

Surveying instruments were continually being improved through the mid-1850s, but a few basic pieces were used by almost every surveyor of the period. Equipment used by George Washington while surveying included: a brass plain (or plane) surveying compass, a jacob staff, a surveyor's chain and poles, an "18 inch Circumferentor with Sights to let down," a lodestone, a twelve inch brass Gunter [scale], full and compleat, one side to have inches and 10ths and the other inches and l2ths as usual," a brass parallel rule, and a case of surveyor's plotting instruments. (Bedini, Thinkers and Tinkers, p. l~4).

Jared Mansfield, surveyor-general for the surveys of the Indiana Territory, ordered his instruments from London: "A three-foot Reflecting Telescope, mounted in the best manner, with powers, lever-motion, Wollaston's Catalogue of the Stars, Mackelyne 's Observations and Tables, A thirty inch Portable Transit Instrument, answering also the purpose of an Equal Altitude Instrument and Therdolete, An Astronomical Pendulum Clock." (Dudley, "Jared Mansfield," p. 238).

The instruments used by colonial and Revolutionary surveyors were generally imported from England. After the Revolution, some instruments were made by skilled metalworkers in the East; the makers of astronomical instruments were usually mathematicians as well. The brass used for instrument-making was entirely imported from England until the second quarter of the 19th century, making even domestic-made instruments quite expensive. Some instrument makers experimented, using wood to replace certain brass parts. Most of the precision instruments produced before the 19th century were navigational devices. As the nation expanded westward, the need for surveying equipment grew, and production gradually shifted from nautical to land-measuring devices. (Bedini, pp. 186-187).

The most basic tools used in surveying were the chain and stakes, jacob staff, plain surveying compass, plane table, and transit; a theodolite often took the place of the compass and transit. The standard measuring chain was of the type called Gunter's chain, consisting of 100 links totaling 66 feet (7.92 inches per link). This was also called a four-pole chain, since it contained four poles (also called "perches" or "rods") of 16½ feet each. (Bedini, pp. 463-~64). These chains were usually made of wrought iron or steel.

In 1796, in an effort to standardize measurements, Congress commissioned David Rittenhouse to construct a surveyor's chain to serve as a standard for the U.S. Land Office. Rittenhouse produced a chain of 80 links measuring 66 feet (9.9 inches per link), made of brass. (Bedini, p. 317). This was presumably the prototype for chains used to measure territory under the system of 1796 (which included Indiana Territory).

The jacob staff was a long, straight wooden rod, usually round, which was used as a base for surveying instruments. One end was steel- pointed and sunk into the ground; the other was fitted with a brass head on which the instrument was mounted. Tripods, or three-legged stands, were also used, and probably provided a firmer base of support for the instruments.

The plain surveying compass ... was relatively easy to produce in wood. It consisted primarily of a circular body flat on the top and bottom, which could be easily turned on a lathe with a cavity inside which would accommodate a compass card and needle which would be protected with a glass pane set in putty. From the central body two arms extended which were slotted at each end to receive sighting bars which were whittled and fitted into place but could be removed for traveling. A turned piece of wood with a circular opening in its center was attached to the center of the bottom of the instrument so that it could be attached to a tripod or jacob staff. The compass card was generally the type used for marine compasses and engraved with the compass rose. A cover was also provided, cut from pine or other inexpensive wood, with leather thongs with which it could be tied over the glass pane of the compass (Bedini, p. 196)

Wooden surveying compasses were apparently produced solely in the New England colonies, and were made in substantial quantities from around 17240 through about 1825. (Bedini, p. 196). Brass was apparently used for most of these instruments produced elsewhere.

The theodolite was a 10- to 12-inch circle of brass having a limb divided into 3600 and again subdivided. A circumferentor (a device with sighting ridges used in measuring angles) was set into this ring, and a compass inset into the circumferentor. In early versions, a pair of sights was attached to turn on the center of the instrument; after 1720 a telescope replaced the sights. This instrument was used to measure angles of the field or the bearing of stationary distance lines from the meridian. Its chief disadvantage, as of the plain surveying compass, was that the sights or telescope could not revolve completely on the axis, limiting the sightings that could be taken before repositioning the device. (Bedini, pp. 2485-2486).

The surveyor's transit was basically a plain surveying compass with the sighting bars replaced with a telescope. This allowed horizontal and vertical angles of an objective to be measured simultaneously. The transit was an improvement over the surveying compass and the theodolite in that the telescope could be completely revolved on its axis. Transits were first produced commercially in about 1830 in Philadelphia, and soon achieved widespread use.

The plane table was a smooth wooden panel which was attached to a jacob staff or tripod. It was equipped with sighting bars on each end and a removable metal bar in the center; one edge was marked into a scale of equal parts, and another with divisions of the semicircle. This table served as a field desk on which maps and charts were made; plotting information obtained from the compass or transit was transferred to the paper clamped in place on the plane table, and the information could be checked with the sights on the table. (Bedini, p. 478).

Other instruments, primarily variations of those listed, were also used, but the listed devices were the ones most commonly used in American surveying to the mid-l9th century