The base units shown in this dataset are used to define all the unit atoms of The Unified Code for Units of Measure according to its grammar and semantics. There are five columns titled “name,” “kind of quantity,” “print,” “c/s,” and “c/i.” The name is the full (official) name of the unit. The official symbol used in print this is listed in the column “print” “C/s,” and “c/i” list the symbol in the case sensitive and the case insensitive variants respectively. Only the columns titled “c/s,” and “c/i,” are normative. Full name and print symbol are defined by other bodies and are out of scope of The Unified Code for Units of Measure. The selection of base units and the particular order are not normative. Any other basis B’ that generates an isomorphic group of units is conformant with The Unified Code for Units of Measure. If the other base B’ generates a different system of units U’ it conforms to The Unified Code for Units of Measure only if there is an homomorphism that maps U’ onto U. Base units must be metric units only. Special units cannot be base units.
As can be seen the base system used to define The Unified Code for Units of Measure is different from the system used by the Système International d’Unités (SI) The SI base unit kilogram has been replaced by gram and the mole has been replaced by the radian that is defined dimensionless in the SI. Because of the latter change, the Unified Code for Units of Measure is not isomorphic with the SI.
The replacement of the kilogram is trivial. In order to bring syntax and semantics in line we can not have a unit with prefix in the base. We need a valid unit of mass before we can combine it with the prefix “kilo-” This change does not have any effect on the semantics whatsoever. The base unit kilogram is one of the oddities of the SI: if the gram would have been chosen as a base units the CGPM (General Conference on Weights and Measures) could have saved the rather annoying exception of the prefixing rules with the kilogram. At times where we have to multiply the wavelength of excited krypton-86 atoms by 1650763.73 to yield one meter, it seems trivial to divide the prototype of the kilogram by thousand to yield a base unit gram.
The rationale for removing the mole from the base is that the mole is essentially a count of particles expressed in a unit of very high magnitude (Avogadro’s number). There is no fundamental difference between the count of particles and the count other things.
The radian has been adopted as the base unit of plane angle α to facilitate the distinction from the solid angle Ω by the relation Ω = α2 and to distinguish rotational frequency f from angular velocity ω = 2 π · rad · f.