Basics of Coordinate Metrology
Unit 1:  Units - Base Quantities

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A distinction is made between base quantities, for which no definition exists in the sense of basing them on quantities already known, and derived quantities, which can be developed from the base quantities. The base quantities are defined by base units, from which the units that apply to derived quantities are obtained.

Since the definition of base quantities and base units is in principle arbitrary, a whole series of different quantity and unit systems have been established in physics over the years ( History of the length units). Therefore, the recommendation by the General Conference for Measurements and Weights in 1960 to make general use of the International System of Units (Systême International d'Unités, metric system) ( Definition of all SI units) was considered a big step forward. The International System of Units is based on the following seven base quantities and base units (see on the right):

Derived quantities, such as the speed, can be defined by means of these base quantities. (Speed = Distance / Time) The units of these derived quantities are products of powers of the base units without numeric factors. (Unit of the speed = meter / second)

     Base quantity Name Symbol
Length Meter

1 m
Mass Kilogram 1 kg
Time Second 1 s
Thermodynamic temperature Kelvin 1 K
Electric current Ampere 1 A
Amount of substance Mole 1 mol
Luminous intensity Candela 1 cd
 


 
Since 1983 the definition of the meter has been as follows:
meter = the length of a path which traveled by light in a vacuum within 1/299.792.458th of a second.