Rod (unit)

The rod, perch, or pole is a surveyor's tool and unit of length of various historical definitions. In British imperial and US customary units, it is defined as feet, equal to exactly of a mile, or yards, and is exactly 5.0292 meters. The rod is useful as a unit of length because integer multiples of it can form one acre of square measure. The 'perfect acre' is a rectangular area of 43,560 square feet, bounded by sides 660 feet long and 66 feet wide or, equivalently, 40 rods by 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 been supplanted, first by steel tapes and later by electronic tools such as surveyor lasers and optical target devices for surveying lands. 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.
In the United States until 1 January 2023, the rod was often defined as 16.5 US survey feet, or approximately 5.029 210 058 m.

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, modern land measures 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 had seized directly from church property, 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 described the measure of an acre in terms of a perch:
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. There are 10 chains or 40 rods in a furlong, and so 80 chains or 320 rods in one statute mile ; the definition of which was legally set in 1593 and popularized by Royal surveyor John Ogilby only after the Great Fire of London.
An acre is defined as the area of 10 square chains, and derives from the shapes of new-tech plows 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 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 with surveys for land sales, canals and railways increasing, surveyor rods such as those used by George Washington were generally made of dimensionally stable metal—semi-flexible drawn wrought iron linkable bar stock so 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, 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 triangulations.
The rod as a survey measure was standardized by Edmund Gunter in England in 1607 as a quarter of a chain, or long.

In ancient cultures

The perch as a lineal measure in Rome was 10 Roman feet, and in France varied from 10 feet to 22 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 and canne,, and pertica, and. They were subdivided in many different ways, and were of many different lengths.

Place	Local name	Local equivalent	Metric equivalent
Aachen	Feldmeßruthe	16 Fuß	4.512
Amsterdam	Roede	13 Voet	3.681
Aubenas, Ardèche	canne	8 pans	1.985
Baden, Grand Duchy of	Ruthe	10 Fuß	3.0
Basel, Canton of	Ruthe	16 Fuß	4.864
Bern, Canton of	Ruthe	10 Fuß	2.932
Barcelona	canna	8 palmos	1.581
Braunschweig	Ruthe	16 Fuß	4.565
Bremen	Ruthe	8 Ellen or 16 Fuß	4.626
Brussels	Ruthe	20 Fuß	4.654
Cagliari, Sardinia	canna	10 palmi	2.322
Calenberg Land	Ruthe	16 Fuß	4.677
Cassel, Hessen	Ruthe	14 Fuß	4.026
Denmark	Ruthe	10 Fuß	3.138
	Ruthe	8 Fuß	2.598
Hamburg	Geestruthe	16 Fuß	4.583
Hamburg	Marschruthe	14 Fuß	4.010
Hannover	Ruthe	16 Fuß	4.671
France	Perche	3 toises	5.847
France	Perche	toises	7.145
Genoa	canna	10 palmi	2.5
Jever, Oldenburg	Ruthe	20 Fuß	4.377
Mallorca	canna	8 palmos	1.714
Malta	canna	8 palmi	2.08
Mecklenburg	Ruthe	16 Fuß	4.655
Menorca, but not Mahón	canna		1.599
Menorca, city of Mahon	canna	8 palmos	1.714
Messina, Sicily	canna	8 palmi	2.113
Montauban, Tarn-et-Garonne	canne	8 pans	1.783
Morocco	canna	8 palmos	1.714
Naples	canna	8 palmi
Naples, Kingdom of: Apulia, Calabria, Eboli, Foggia, Lucera	percha	7 palmi	1.838
Naples, Kingdom of: Capua	percha	palmi	1.892
Naples, Kingdom of: Fiano, Naples	percha	palmi	2.014
Naples, Kingdom of: Caggiano, Cava, Nocera, Rocce, Salerno	percha	palmi	1.971
Nuremberg, Bavaria	Ruthe	16 Fuß	4.861
Oldenburg	Ruthe	20 Fuß	5.927
Palermo, Sicily	canna	8 palmi	1.942
Parma	Pertica	6 bracci	3.25
Poland	Pręt	łokci or 10 pręcików	4.320
Prussia, Rheinland	Ruthe	12 Fuß	3.766
Rijnland	Roede	12 Voet	3.767
Rome	canna		2
Rome	canna		2.234
Saragoza	canna		2.043
Saxony	Ruthe	16 Leipziger Fuß	4.512
Sweden	Ruthe	16 Fuß	4.748
Tortosa	canna		1.7
Tuscany, Grand-Duchy of	canna	5 bracci	2.918
Uzès, Gard	canne	8 pans	1.98
Waadt, Canton of	Ruthe or toise courante	10 Fuß	3
Württemberg	Reichsruthe	10 Fuß	2.865
Württemberg	old Ruthe	16 Fuß	4.583
Venice, Republic of	Pertica	6 piedi	2.084
Zürich, Canton of	Ruthe	10 Fuß	3.009

In Britain and Ireland

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.
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 by one furlong.
Bars of metal one rod long were used as standards of length when surveying land. The rod was still in use as a common unit of measurement in the mid-19th century, when Henry David Thoreau used it frequently when describing distances in his work, Walden.
In traditional Scottish units, a Scottish rood, also fall measures 222 inches.