While all compasses measure horizontal angles with reference to magnetic
north, a surveyor’s compass is equipped with vertical sights for taking aim at
distant objects. The surveyor’s compass was originally a colonial instrument,
designed for use in places such as Ireland and America where land was plentiful. The first reference to an instrument of this sort appeared in William Folkingham, Art of Surveying (London, 1610). The earliest extant example was made in Dublin in 1667, and is now in the Museum of the History of Science, Oxford. The terms surveyor’s compass and surveying compass were in use in America by the 1750s; English texts refer to this instrument as a circumferentor. When vernier compasses came into use, the simpler instruments became known as plain compasses.
A vernier compass is a compass with a variation arc and a vernier mechanism. With it, a surveyor can compensate for magnetic variation, and thus run lines in relation to the true meridian, or retrace old lines (assuming that he knew the extent of magnetic variation at the time the line was originally run). The form originated in the United States. David Rittenhouse made one of the earliest instruments of this sort, probably in the early 1780s. Benjamin Rittenhouse made numerous vernier compasses in the period 1785–1800. In 1798, Surveyor General of the United States Rufus Putnam told a prospective surveyor to obtain "a compass having a moveable band (Mr. Rittenhouse, near Philadelphia, makes the best I have seen.)." In 1804, deputy surveyors were informed that fieldwork was to be done with a "Rittenhouse compass with a Nonius & a common 2 pole chain of 50 links." (A vernier was also known as a nonius.) W. & L. E. Gurley introduced the term vernier compass in the 1850s.
In 1830, William J. Young applied for a patent on an "Improved Surveying Compass" designed to measure horizontal angles either with or without reference to magnetic north. Young’s design proved to be especially useful for railroad surveys, and most examples were sold for that purpose. An advertisement in the American Railroad Journal for March 23, 1833, carried several testimonials. One from a civil engineer stated: "Having for the last two years made constant use of Mr. Young’s ‘Patent Improved Compass,’ I can safely say I believe it to be much superior to any other instrument of the kind, now in use, and as such most cheerfully recommend it to Engineers and Surveyors." The term "Railroad Compass" came into use after the expiration of Young’s patent in the mid-1840s.
Most surveyor’s compasses are made of brass, but wooden compasses with paper cards that had been designed for nautical use were fairly common in New England in the 18th and early 19th centuries. Those carrying the names of some 25 instrument makers and dealers are known. Wooden compasses would not have been very accurate, but were probably adequate at a time when brass was scarce and land was plentiful.
The face of most American compasses reads counterclockwise. That means that when a surveyor sights from the south to the north end of the instrument, the needle points to the direction he/she is looking. David Rittenhouse introduced this design to America in the 1760s. There are similar Irish instruments from a bit earlier. The face of most English compasses reads clockwise. Some compasses are equipped with an outkeeper, a dial that helps a surveyor keep track of the number of times the chain has been run (these are known as outs). Some have a dial that converts outs to poles (a pole, also known as a rod or a perch, is equal to 5.5 yards).