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. In dialectal English the term lug has also been used, although the Oxford English Dictionary states that this unit, while usually of 16 + 1⁄ 2 feet, may also be of 15, 18, 20, or 21 feet. Surveyors rods and chains are still used in rough terrains with heavy overgrowth where laser or other optical measurements are difficult or impossible. The tool has largely been supplanted by electronic tools such as surveyor lasers ( lidar) and optical target devices for surveying lands. Both measures date from the sixteenth century, when the pike was still utilized in national armies. The measure also has a relationship with the military pike of about the same size. The name perch derives from the Ancient Roman unit, the pertica. An acre is therefore 160 square rods or 10 square chains. The 'perfect acre' is a rectangular area of 43,560 square feet, bounded by sides 660 feet (a furlong) long and 66 feet (a chain) wide (220 yards by 22 yards) or, equivalently, 40 rods by 4 rods. The rod is useful as a unit of length because integer multiples of it can form one acre of square measure (area). 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. The rod, perch, or pole (sometimes also lug) is a surveyor's tool and unit of length of various historical definitions. Not to be confused with a surveyor's pole, upon which a survey instrument is mounted a ranging rod used for sighting or, a level staff, which may also be called a leveling rod.
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