Unit of length: metre (m)

​The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency  ∆νCs.
​This definition implies the exact relation c = 299 792 458 m s–1.


Practical realization of the definition of the metre

The mise en pratique of the definition of the metre specifies that the metre should be realized by one of the following methods:

  • by means of the length of the path travelled in vacuum by a plane electromagnetic wave in a time t,
  • by means of the wavelength in vacuum of a plane electromagnetic wave of frequency f; this wavelength is obtained from the measured frequency f using the relation = c0/f ,
  • by means of one of the listed recommended radiations, whose stated wavelength in vacuum or whose stated frequency can be used with the uncertainty shown, provided that the given specifications and accepted good practice are followed.


Unit of plane angle: radian (rad)radian.png

The radian is determined by the circumference of a circle that is equal in length to the radius of the circle.

The SI-unit for angle is radian (rad) but depending on the branch of industry and the measurement subject other units are commonly used. In mechanical engineering angles are normally expressed in degrees [°], minutes ['] and seconds ["], in geodesy the most commonly used unit is gon [gon] (also referred to as grade). In earth-moving work and for small angles the unit [mm/m] is generally used. The diverse group of units for angle include also the following ways to express the angle: percent [%] and length ratios.

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