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Imperial units or the Imperial system is a collection of units, first defined in the British Weights and Measures Act of 1824, later refined (until 1959) and reduced. The units were introduced in the United Kingdom and its colonies, including Commonwealth countries (most have since become officially metric, but continue to use both Metric and Imperial), but excluding the then already independent United States. Systems of imperial units are sometimes referred to as foot-pound-second, after the base units of length, mass and time. The Imperial System is becoming more and more obsolete. It is only officially used in three countries. Although most of the units are defined in more than one system, some subsidiary units were used to a much greater extent, or for different purposes, in one area rather than the other. After the 1 July 1959 deadline, agreed upon in 1958, the U.S. and the British yard were defined identically, at 0.9144 m to match the international yard. Metric equivalents in this article usually assume this latest official definition. Before this date, the most precise measurement of the imperial Standard Yard was 0.914398416 m (Sears et al. 1928. Phil Trans A 227281). The fathom is not in use any more. The British Admiralty in practice used a fathom as 6 feet. This was despite it being 1/1000 of a nautical mile (i.e. 6.08 feet) until 1970, when the international nautical mile of exactly 1852 metres was adopted. The commonly accepted definition of a fathom was always 6 feet. The conflict was inconsequential in determining depth as Admiralty nautical charts used feet as depths below 5 fathoms on older imperial charts. Today all charts worldwide are metric, except for USA Hydrographic Office charts, which use feet only for all depth ranges.
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