Rod (unit)

The rod, perch, or pole (sometimes also lug) is a surveyor's tool 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. The rod is useful as a unit of length because integer 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 (a chain) wide (220 yards by 22 yards) 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 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 16+1⁄2 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.