Perch (unit)
| rod | |
|---|---|
| Unit system | imperial/US units |
| Unit of | length |
| 1 rod in ... | ... is equal to ... |
| imperial/US units | 5 1⁄2 yd |
| metric (SI) units | 5.0292 m |
The rod or perch or pole is a surveyors tool and unit of length equal to 5 1⁄2 yards, 16 1⁄2 feet, 1⁄320 of a statute mile or one-fourth of a surveyor's chain and 5.0292 meters. The rod is useful as a unit of length because whole number multiples of it can equal one acre of square measure. The 'perfect acre' is a rectangular area of 43,560 square feet, bounded by sides 660 feet by 66 feet long (660 ft long × 66 ft wide), or 220 yards by 22 yards long (220 yd/ long × 22 yd wide), or 40 rods by 4 rods long. Thus, an acre is 160 square rods. Since the adoption of the international yard on 1 July 1959, the rod has been equal to exactly 5.0292 meters.
A rod is the same length as a perch, also sometimes called a pole which measure using cordage or wood, slightly antedated the use of both rods and surveyors chains, made of more dimensionally regular materials. Its name derives from the Ancient Roman unit, the pertica. The measure also has a relationship to the military pike of about the same size and both measures date from the sixteenth century, when that weapon was still utilized in national armies. The tool, normally configured as a metal rod with eye-ends (loops that could be hooked together), was used commonly until quite recently, when it was supplanted by electronic tools such as surveyor lasers (Lidar) and optical target devices for surveying lands. Surveyors rods and chains are still utilized in rough terrains with heavy overgrowth where laser or other optical measurements are difficult or impossible. In old English, the term lug is also used.
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 18 feet (5.49 m), 20 feet (6.1 m), 22 feet (6.71 m) and 24 feet (7.32 m); and even as late as 1820, a House of Commons report notes lengths of 16 1⁄2 feet (5.03 m), 18 feet (5.49 m), 21 feet (6.4 m), 24 feet (7.32 m), and even 25 feet (7.62 m). In Ireland, a perch was standardized at 21 feet (6.4 m), making an Irish chain, furlong and mile proportionately longer by 27.27% than the "standard" English measure.
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