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The link (usually abbreviated as "l.", "li." or "lnk."), sometimes called a Gunter’s link, is a unit of length formerly used in many English-speaking countries. In US customary units modern definition, the link is exactly 66 ⁄ 100 of a US survey foot , [ 1 ] or exactly 7.92 inches or 20.1168 cm.
The rod, perch, or pole (sometimes also lug) is a surveyor's tool [1] and unit of length of various historical definitions. In British imperial and US customary units , it is defined as 16 + 1 ⁄ 2 feet , equal to exactly 1 ⁄ 320 of a mile , or 5 + 1 ⁄ 2 yards (a quarter of a surveyor's chain ), and is exactly 5.0292 meters.
Surveying prism pole with instrument attached. In surveying, a pole is bar made of wood or metal and normally held vertical, upon which different instruments can be mounted: a prism, a GPS device, etc. [1] It may be manufactured with a predetermined length (e.g., 2 meters) or may be graduated for different heights or stages.
For most applications, the difference between the two definitions is insignificant – one international foot is exactly 0.999 998 of a US survey foot, for a difference of about 1 ⁄ 8 in (3 mm) per mile – but it affects the definition of the State Plane Coordinate Systems (SPCSs), which can stretch over hundreds of miles.
Each survey consists of approximately 1,000 completed interviews among U.S. adults using a sample selected from YouGov’s opt-in online panel of all 50 states plus the District of Columbia to match the demographics and other characteristics of the adult U.S. population.
The 66-foot (20.1 m) chain is divided into 100 links, usually marked off into groups of 10 by brass rings or tags which simplify intermediate measurement. Each link is thus 7.92 inches (201 mm) long. A quarter chain, or 25 links, measures 16 feet 6 inches (5.03 m) and thus measures a rod (or pole).
A benchmark is usually set on the side, marked with the letters "O S B M" (Ordnance Survey Bench Mark) and the reference number of the trig point on a plaque called a "flush plate". Within and below the visible trig point, there are concealed reference marks whose National Grid References are precisely known.
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