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The unit used in the US is the foot sea water (fsw), based on standard gravity and a sea-water density of 64 lb/ft 3. According to the US Navy Diving Manual, one fsw equals 0.30643 msw, 0.030 643 bar, or 0.444 44 psi, [1] [2] though elsewhere it states that 33 fsw is 14.7 psi (one atmosphere), which gives one fsw equal to about 0.445 psi. [3]
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.
15.24 meters – width of an NBA basketball court (50 feet) 18.44 meters – distance between the front of the pitcher's rubber and the rear point of home plate on a baseball field (60 feet, 6 inches) [125] 20 meters – length of cricket pitch (22 yards) [126] 27.43 meters – distance between bases on a baseball field (90 feet)
A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. [2] [3] [4] Historically, it was defined as the meridian arc length corresponding to one minute ( 1 / 60 of a degree) of latitude at the equator, so that Earth's polar circumference is very near to 21,600 nautical miles (that is 60 minutes × 360 degrees).
Miners also use it as a unit of area equal to 6 feet square (3.34 m 2) in the plane of a vein. [2] In Britain, it can mean the quantity of wood in a pile of any length measuring 6 feet (1.8 m) square in cross section. [2] In Central Europe, the klafter was the corresponding unit of comparable length, as was the toise in France.
The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458 of a second, where the second is defined by a hyperfine transition frequency of caesium.
In 2020, the National Institute of Standards and Technology announced that the U.S. survey foot would "be phased out" on 1 January 2023 and be superseded by the international foot (also known as the foot) equal to 0.3048 metres exactly, for all further applications. [48] This implies that the survey inch was replaced by the international inch.
A geoid is needed to convert the 3D position to sea-level elevation. Pressure altimeter measuring atmospheric pressure, which decreases as altitude increases. Since atmospheric pressure varies with the weather, too, a recent local measure of the pressure at a known altitude is needed to calibrate the altimeter. Stereoscopy in aerial photography.