Foot (measure)

The foot (standard symbol: ft) is a unit of length in the British imperial and United States customary systems of measurement. The prime symbol, , is commonly used to represent the foot. In both customary and imperial units, one foot comprises 12 inches, and one yard comprises three feet. Since an international agreement in 1959, the foot is defined as equal to exactly 0.3048 meters.

Historically, the "foot" was a part of many local systems of units, including the Greek, Roman, Chinese, French, and English systems. It varied in length from country to country, from city to city, and sometimes from trade to trade. Its length was usually between 250 mm and 335 mm and was generally, but not always, subdivided into 12 inches or 16  digits.

The United States is the only industrialized country that uses the (international) foot in preference to the meter in its commercial, engineering, and standards activities. The foot is legally recognized in the United Kingdom; road distance signs ''must'' use imperial units (however, distances on road signs are always marked in miles or yards, not feet; bridge clearances are given in meters as well as feet and inches), while its usage is widespread among the British public as a measurement of height. The foot is recognized as an alternative expression of length in Canada. Both the UK and Canada have partially metricated their units of measurement. The measurement of altitude in international aviation (the flight level unit) is one of the few areas where the foot is used outside the English-speaking world.

The most common plural of foot is feet. However, the singular form may be used like a plural when it is preceded by a number, as in "he is six foot tall."

Historical origin

Historically, the human body has been used to provide the basis for units of length. The foot of an adult European-American male is typically about 15.3% of his height, giving a person of a foot-length of about , on average.

Archaeologists believe that, in the past, the people of Egypt, India, and Mesopotamia preferred the cubit, while the people of Rome, Greece, and China preferred the foot. Under the Harappan linear measures, Indus cities during the Bronze Age used a foot of and a cubit of . The Egyptian equivalent of the foot—a measure of four palms or 16 digits—was known as the and has been reconstructed as about .

The Greek foot (, ) had a length of of a stadion, one stadion being about ; therefore a foot was, at the time, about . Its exact size varied from city to city and could range between and , but lengths used for temple construction appear to have been about to ; the former was close to the size of the Roman foot.

The standard Roman foot () was normally about (97% of today's measurement),Hosch, William L. (ed.) (2010
''The Britannica Guide to Numbers and Measurement''
New York, NY: Britannica Educational Publications, 1st edition. , p.206 but in some provinces, particularly Germania Inferior, the so-called (foot of Nero Claudius Drusus) was sometimes used, with a length of about . (In reality, this foot predated Drusus.)

Originally both the Greeks and the Romans subdivided the foot into 16 digits, but in later years, the Romans also subdivided the foot into 12 (from which both the English words "inch" and "ounce" are derived).

After the fall of the Roman Empire, some Roman traditions were continued but others fell into disuse. In AD 790 Charlemagne attempted to reform the units of measure in his domains. His units of length were based on the and in particular the , the distance between the fingertips of the outstretched arms of a man. The has 6 (feet) each of .

He was unsuccessful in introducing a standard unit of length throughout his realm: an analysis of the measurements of Charlieu Abbey shows that during the 9th century the Roman foot of was used; when it was rebuilt in the 10th century, a foot of about The original reference was given in a round number of centimeters. was used. At the same time, monastic buildings used the Carolingian foot of .

The procedure for verification of the foot as described in the 16th century posthumously published work by Jacob Köbel in his book is:

England

The Neolithic long foot, first proposed by archeologists Mike Parker Pearson and Andrew Chamberlain, is based upon calculations from surveys of Phase 1 elements at Stonehenge. They found that the underlying diameters of the stone circles had been consistently laid out using multiples of a base unit amounting to 30 long feet, which they calculated to be 1.056 of a modern international foot (thus 12.672 inches or 0.3219 m). Furthermore, this unit is identifiable in the dimensions of some stone lintels at the site and in the diameter of the "southern circle" at nearby Durrington Walls. Evidence that this unit was in widespread use across southern Britain is available from the Folkton Drums from Yorkshire (neolithic artifacts, made from chalk, with circumferences that exactly divide as integers into ten long feet) and a similar object, the Lavant drum, excavated at Lavant, Sussex, again with a circumference divisible as a whole number into ten long feet.

The measures of Iron Age Britain are uncertain and proposed reconstructions such as the Megalithic Yard are controversial. Later Welsh legend credited Dyfnwal Moelmud with the establishment of their units, including a foot of 9 inches. The Belgic or North German foot of was introduced to England either by the Belgic Celts during their invasions prior to the Romans or by the Anglo-Saxons in the 5th and 6th century.

Roman units were introduced following their invasion in AD 43. Following the Roman withdrawal and Saxon invasions, the Roman foot continued to be used in the construction crafts while the Belgic foot was used for land measurement. Both the Welsh and Belgic feet seem to have been based on multiples of the barleycorn, but by as early as 950 the English kings seem to have (ineffectually) ordered measures to be based upon an iron yardstick at Winchester and then London. Henry I was said to have ordered a new standard to be based upon the length of his own arm and, by the Act concerning the Composition of Yards and Perches traditionally credited to Edward I or II, the statute foot was a different measure, exactly of the old (Belgic) foot. The barleycorn, inch, ell, and yard were likewise shrunk, while rods and furlongs remained the same. The ambiguity over the state of the mile was resolved by the 1593 Act against Converting of Great Houses into Several Tenements and for Restraint of Inmates and Inclosures in and near about the City of London and Westminster, which codified the statute mile as comprising 5,280 feet. The 1959 adoption of the international foot completed a redefinition of the foot in terms of the meter.

Definition

International foot

The international yard and pound agreement of July 1959 defined the length of the international yard in the United States and countries of the Commonwealth of Nations as exactly 0.9144 meters. Consequently, since a foot is one third of a yard, the international foot is defined to be equal to exactly 0.3048 meters. This was 2  ppm shorter than the previous US definition and 1.7 ppm longer than the previous British definition.

The 1959 agreement concluded a series of step-by-step events, set off in particular by the British Standards Institution's adoption of a scientific standard inch of 25.4 millimeters in 1930.

Symbol

The IEEE standard symbol for a foot is "ft". In some cases, the foot is denoted by a prime, often approximated by an apostrophe, and the inch by a double prime; for example, 2feet 4 inches is sometimes denoted 2′4″.

Imperial units

In Imperial units, the foot was defined as  yard, with the yard being realized as a physical standard (separate from the standard meter). The yard standards of the different Commonwealth countries were periodically compared with one another. The value of the United Kingdom primary standard of the yard was determined in terms of the meter by the National Physical Laboratory in 1964 to be , implying a pre-1959 UK foot of .

The UK adopted the international yard for all purposes through the Weights and Measures Act 1963, effective January 1, 1964.

Survey foot

When the international foot was defined in 1959, a great deal of survey data was already available based on the former definitions, especially in the United States and in India. The small difference between the survey foot and the international foot would not be detectable on a survey of a small parcel, but becomes significant for mapping, or when the state plane coordinate system (SPCS) is used in the US, because the origin of the system may be hundreds of thousands of feet (hundreds of miles) from the point of interest. Hence the previous definitions continued to be used for surveying in the United States and India for many years, and are denoted survey feet to distinguish them from the international foot. The United Kingdom was unaffected by this problem, as the retriangulation of Great Britain (1936–62) had been done in meters.

US survey foot

In the United States, the foot was defined as 12 inches, with the inch being defined by the Mendenhall Order of 1893 via 39.37 inches = 1 m (making a US foot exactly meters, approximately ).

On December 31, 2022, the National Institute of Standards and Technology, the National Geodetic Survey, and the United States Department of Commerce deprecated use of the US survey foot and recommended conversion to either the meter or the international foot (0.3048 m). However, the historic relevance of the US survey foot persists, as the ''Federal Register'' notes:

State legislation is also important for determining the conversion factor to be used for everyday land surveying and real estate transactions, although the difference (two parts per million) is of no practical significance given the precision of normal surveying measurements over short distances (usually much less than a mile). Out of 50 states and six other jurisdictions, 40 have legislated that surveying measures should be based on the US survey foot, six have legislated that they be made on the basis of the international foot, and ten have not specified.

Indian survey foot

The Indian survey foot is defined as exactly , presumably derived from a measurement of the previous Indian standard of the yard. The current National Topographic Database of the Survey of India

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