Rod (unit)

The rod, perch, or pole (sometimes also lug) is a surveyor's tool and unit of length of various historical definitions, often between approximately 3 and 8 meters (9 ft 10 in and 26 ft 2 in). In modern US customary units it is defined as US survey feet, equal to exactly of a surveyor's mile, or a quarter of a surveyor's chain ( yards), and is approximately 5.0292 meters. The rod is useful as a unit of length because whole number multiples of it can form one acre of square measure (area). The 'perfect acre' is a rectangular area of 43,560 square feet, bounded by sides 660 feet (a furlong) long and 66 feet wide (220 yards by 22 yards) or, equivalently, 40 rods and 4 rods. An acre is therefore 160 square rods or 10 square chains.

The name ''perch'' derives from the Ancient Roman unit, the ''pertica''.
The measure also has a relationship with the military pike of about the same size. Both measures date from the sixteenth century, when the pike was still utilized in national armies. The tool has largely been supplanted by electronic tools such as surveyor lasers (lidar) and optical target devices for surveying lands. Surveyors rods and chains are still used in rough terrains with heavy overgrowth where laser or other optical measurements are difficult or impossible. In dialectal English the term ''lug'' has also been used, although the ''Oxford English Dictionary'' states that this unit, while usually of feet, may also be of 15, 18, 20, or 21 feet.

History

In England, the perch was officially discouraged in favour of the rod as early as the 15th century; however, local customs maintained its use. In the 13th century perches were variously recorded in lengths of , , and ; and even as late as 1820, a House of Commons report notes lengths of , , , , and even . In Ireland, a perch was standardized at , making an Irish chain, furlong and mile proportionately longer by 27.27% than the "standard" English measure.

Until English King Henry VIII seized the lands of the Roman Catholic Church in 1536, land measures as we now know them were essentially unknown. Instead a narrative system of landmarks and lists was used. Henry wanted to raise even more funds for his wars than he'd seized directly from church property (he'd also assumed the debts of the monasteries), and as James Burke writes and quotes in the book ''Connections'' that the English monk Richard Benese "produced a book on how to survey land using the simple tools of the time, a rod with cord carrying knots at certain intervals, waxed and resined against wet weather." Benese poetically described the measure of an acre in terms of a perch:''Connections'', pbk. p.263

The practice of using surveyor's chains, and perch-length rods made into a detachable stiff chain, came about a century later when iron was a more plentiful and common material. A chain is a larger unit of length measuring , or 22 yards, or 100 links, or 4 rods (20.1168 meters). There are 10 chains or 40 rods in a furlong (eighth-mile), and so 80 chains or 320 rods in one statute mile (1760 yards, 1609.344 m, 1.609344 km); the definition of which was legally set in 1593 and popularized by Royal surveyor (called the 'sworn viewer'"Connections", pbk. p.265) John Ogilby only after the Great Fire of London (1666).

An acre is defined as the area of 10 square chains (that is, an area of one chain by one furlong), and derives from the shapes of new-tech plowsConnections, pbk. p.63 and the desire to quickly survey seized church lands into a quantity of squares for quick sales by Henry VIII's agents; buyers simply wanted to know what they were buying whereas Henry was raising cash for wars against Scotland and France. Consequently, the surveyor's chain and surveyor rods or poles (the perch) have been used for several centuries in Britain and in many other countries influenced by British practices such as North America and Australia. By the time of the industrial revolution and the quickening of land sales, canal and railway surveys, et al. Surveyor rods such as used by George Washington were generally made of dimensionally stable metal—semi-flexible drawn wrought iron linkable bar stock (not steel), such that the four folded elements of a chain were easily transportable through brush and branches when carried by a single man of a surveyor's crew. With a direct ratio to the length of a surveyor's chain and the sides of both an acre and a square (mile), they were common tools used by surveyors, if only to lay out a known plottable baseline in rough terrain thereafter serving as the reference line for instrumental ( theodolite) triangulations.

The rod as a survey measure was standardized by Edmund Gunter in England in 1607 as a quarter of a chain (of ), or long.

In ancient cultures

The perch as a lineal measure in Rome (also ''decempeda'') was 10 Roman feet (2.96 metres), and in France varied from 10 feet (''perche romanie'') to 22 feet (''perche d'arpent''—apparently of "the range of an arrow"—about 220 feet). To confuse matters further, by ancient Roman definition, an arpent equalled 120 Roman feet. The related unit of square measure was the ''scrupulum'' or ''decempeda quadrata'', equivalent to about .

In continental Europe

Units comparable to the perch, pole or rod were used in many European countries, with names that include french: perche and ''canne'', german: Ruthe, it, canna and ''pertica'', pl, pręt and es, canna. They were subdivided in many different ways, and were of many different lengths.

Based on data from the following:
*^N - Niemann ( Quedlinburg and Leipzig - 1830).

In Britain

In England, the rod or perch was first defined in law by the Composition of Yards and Perches, one of the statutes of uncertain date from the late 13th to early 14th centuries: ''tres pedes faciunt ulnam, quinque ulne & dimidia faciunt perticam'' (three feet make a yard, five and a half yards make a perch).

The length of the ''chain'' was standardized in 1620 by Edmund Gunter at exactly four rods. Fields were measured in '' acres'', which were one chain