Units of Length
| Unit | Divisions | SI Equivalent |
|---|---|---|
| Exact relationships shown in boldface | ||
| International | ||
| 1 point (p) | 352.777778 µm | |
| 1 pica (P̸) | 12 p | 4.233333 mm |
| 1 inch (in) | 6 P̸ | 2.54 cm |
| 1 foot (ft) | 12 in | 0.3048 m |
| 1 yard (yd) | 3 ft | 0.9144 m |
| 1 mile (mi) | 5280 ft or 1760 yd | 1.609344 km |
| US Survey | ||
| 1 link (li) | 33⁄50 ft or 7.92 in | 0.2012 m |
| 1 (survey) foot (ft) | 1200⁄3937 m | 0.30480061 m |
| 1 rod (rd) | 25 li or 16.5 ft | 5.02921 m |
| 1 chain (ch) | 4 rd | 20.11684 m |
| 1 furlong (fur) | 10 ch | 201.1684 m |
| 1 survey (or statute) mile (mi) | 8 fur | 1.609347 km |
| 1 league (lea) | 3 mi | 4.828042 km |
| Nautical | ||
| 1 fathom (ftm) | 2 yd | 1.8288 m |
| 1 cable (cb) | 120 ftm or 1.091 fur | 219.456 m |
| 1 nautical mile (NM or nmi) | 8.439 cb or 1.151 mi | 1.852 km |
The system for measuring length in the United States customary system is based on the inch, foot, yard, and mile, which are the only four customary length measurements in everyday use. Since July 1, 1959, these have been defined on the basis of 1 yard = 0.9144 meters except for some applications in surveying. This definition was agreed with the UK and other Commonwealth countries, and so is often termed international measure.
When international measure was introduced in the English-speaking countries, the basic geodetic datum in North America was the North American Datum of 1927 (NAD27), which had been constructed by triangulation based on the definition of the foot in the Mendenhall Order of 1893, that is 1 foot = 1200⁄3937 meters: this definition was retained for data derived from NAD27, but renamed the U.S. survey foot to distinguish it from the international foot. For most applications, the difference between the two definitions is insignificant — one international foot is exactly 0.999998 of a U.S. survey foot, for a difference of about 1⁄8 inch (3 mm) per mile — but it affects the definition of the State Plane Coordinate Systems (SPCSs), which can stretch over hundreds of miles.
The NAD27 was replaced in the 1980s by the North American Datum of 1983 (NAD83), which is defined in meters. The SPCSs were also updated, but the National Geodetic Survey left the decision of which (if any) definition of the foot to use to the individual states. All SPCSs are defined in meters, but seven states also have SPCSs defined in U.S. survey feet and an eighth state in international feet: the other 42 states use only meter-based SPCSs.
State legislation is also important for determining the conversion factor to be used for everyday land surveying and real estate transactions, although the difference (2 ppm) is of no practical significance given the precision of normal surveying measurements over short distances (usually much less than a mile). Twenty-four states have legislated that surveying measures should be based on the U.S. survey foot, eight have legislated that they be made on the basis of the international foot, and eighteen have not specified the conversion factor from metric units.
Read more about this topic: United States Customary Units
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