Foot

A foot (plural: feet or foot; symbol or abbreviation: ft or, sometimes, ′ – a prime) is a unit of length, in a number of different systems, including English units, Imperial units, and United States customary units. more...

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Its size can vary from system to system, but in each is around a quarter to a third of a metre. The most commonly used foot today is the international foot. There are 3 feet in a yard and 12 inches in a foot.

Definition

International foot

In 1958 the United States and countries of the Commonwealth of Nations defined the length of the international yard to be 0.9144 metres. Consequently, the international foot is defined to be equal to 0.3048 metres (equivalent to 304.8 millimetres).

The international standard symbol for a foot is \"ft\" (see ISO 31-1, Annex A). In some cases, the foot is denoted by a prime, which is often approximated by an apostrophe, and the inch by a double prime. For example, 5 feet 2 inches is denoted by 5′2″. This use can cause confusion, because the prime and double prime are also international standard symbols for arcminutes and arcseconds.

United States survey foot

The United States survey foot is defined as exactly ¹²⁰⁰⁄₃₉₃₇ metres, approximately 0.30480061 m. It is used only in connection with surveys by the U.S. Coast and Geodetic Survey, and is officially required by the State Plane Coordinate systems of several states. It is 610 nm greater than the international foot. The U.S. Survey Foot is used by Land Surveyors and other cartographers for the plans and maps they produce. Each state has a law that states which form of the foot is used for surveys within the given state. The difference is particularly noticeable when converting coordinates that are on the State Plane Coordinate System of the given state.

Historical origin

The foot as a measure was used in almost all western cultures and was usually divided into 12, sometimes 10 inches/thumbs or into 16 fingers/digits. The first known standard foot measure was from Sumer, where a definition is given in a statue of Gudea of Lagash from around 2575 BC. Some metrologists speculate that the imperial foot was adapted from an Egyptian measure by the Greeks, with a subsequent larger foot being adopted by the Romans.

Some of the earliest records of the use of the foot come from the region of ancient Greece. The originators devised, or perhaps borrowed from Egypt, the degree of longitude, divided the circumference of the earth into 360 degrees, and subdivided the degree for shorter distances. One degree of longitude comprised 600 stadia. One stadion was divided into 600 feet. Thus the degree of longitude measured 360,000 feet. One mile was 10 stadia or 6000 feet. This is essentially the same mile that was (or still is) used in the Western hemisphere, but the modern foot is longer than the original. This could be explained by an ancient Egyptian measure of the degree of longitude made near Thebes compared to a redefinition of the length of the foot referencing the degree of longitude at the equator. The difference in the length of the geodesic foot measured at these two locations would give the modern mile, 6000 ancient Greek feet or 10 stadia, and 5280 equatorial feet.

Read more at Wikipedia.org