A unit of length is any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary units are also in use. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.
The base unit in the International System of Units (SI) is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 seconds." It is approximately equal to 1.0936 yd. Other SI units are derived from the metre by adding prefixes, as in millimetre or kilometre, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, a kilometre is 1000 m.
In the centimeter–gram–second system of units, the basic unit of length is the centimeter, or 1⁄100 of a meter.
Other non-SI units are derived from decimal multiples of the meter.
The basic unit of length in the imperial and U.S. customary systems is the yard, defined as exactly 0.9144 m by international treaty in 1959.
Common imperial units and U.S. customary units of length include:
thou or mil (1⁄1000 of an inch)
(terrestrial) mile (5280 feet, or 1760 yards 1609.344 m)
(land) league 3 miles (4,800 m)
In addition, the following are used by sailors:
fathom (for depth; only in non-metric countries) (2 yards = 1.8288 m)
nautical mile (one minute of arc of latitude = 1852 m)
Air traffic control uses feet for altitude worldwide, except in a minority of countries including Russia, China, North Korea, and many CIS countries. Distance is measured in nautical miles.
Surveyors in the United States continue to use:
chain (22 yards, or 20.1168 m)
rod (also called pole or perch) (quarter of a chain, 51⁄2 yards, or 5.0292 m)
The Australian building trades adopted the metric system in 1966 and the units used for measurement of length are meters (m) and millimeters (mm). Centimeters (cm) are avoided as they cause confusion when reading plans. For example, the length two and a half meters is usually recorded as 2500 mm or 2.5 m; it would be considered non-standard to record this length as 250 cm.
American surveyors use a decimal-based system of measurement devised by Edmund Gunter in 1620. The base unit is Gunter's chain of 66 feet (20 m) which is subdivided into 4 rods, each of 16.5 ft or 100 links of 0.66 feet. A link is abbreviated "lk", and links "lks", in old deeds and land surveys done for the government.