The American System of measures or US Customary unit system is substantially the same as the imperial system. However, there are some diferences, and this unit system ahcieved the same level of criticisms of inconsistency and complexity of calculations as the imperial unit system. This unit system is still fully legal in the USA and Canada. Where there are differences between them and imperial units, the designation (US) is normally applied. The US weights and measures are based on units used in Britain prior to 1824, when the imperial system was officially established. The US law of 1866 establisehd a relationship with the metric system by defining the metre as equal to 39.37 [in], and, in 1883, the yard was also defined in terms of the metre \(\frac{3600}{3937} [\mathrm{m}]\). The US yard, pound, and all other derived units were redefined in terms of metric units of length and mass, so from that point on there was no longer a direct relation with the UK unit system. An agreement between English speaking countries in 1959, unified the metric definitions of units for scientific and technical uses, with the yard defined as 0.9144 [m] and the pound as 0.45359237 [kg]. To accomodate data from the US geodetic surveys, the old standard of \(1 [\mathrm{ft}] = \frac{1200}{3937} [\mathrm{m}]\) was retained with the name US survey foot . The following relationships with other units are given in Table 1.
The US unit system consist of:
- US customary units of length - as with their british equivalents, the many units of lenght used at varius in the USA fall into several categories and these are:
- US Lenght Units (Linear Measure)
- US Nautical Lenght Units (Nautical Measure)
- US Surveyors' Units (Surveyors' Measure)
- US customary units of area - the US square measure is virtually indetical to the British system and consist of:
- US Area Units
- US Surveyors' Area Units (Surveyors' Measure)
- US Units of Volume and Capacity - As in the case of UK units of length, area, volume and capacity are often associated with specific trades or fields of application. The US units of volume and capacity have similar structure as many older system meaning that there are three categories: geometric measure, expressed in cubic linear units (e.g. cubic foot), dry measure, and liquid measure. The dry and liquid measure categories are known as units of capacity and consist of arbitrary volumes given specific names (e.g. gushel, gallon), with the same name sometimes being used for measuring solids or liquids and thus having different volumes. As stated the US units of volume and capacity have three unit system categories and these are:
- US geometric volume units
- US Volume (capacity) units for liquid substance
- US Volume (capacity) units for dry substance.
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