The foundation of physics rests upon physical quantities in conditions of which the laws of physics are expressed. Therefore, these quantities have to be deliberate accurately. Among these are mass, length, time, velocity, force, density, temperature, electric current, plus numerous others.
Physical quantities are frequently divided into two categories:
Base quantities plus derived quantities. Derived quantities are those whose definitions are based on other physical quantities. Velocity, acceleration plus force etc. are typically viewed as derived quantities. Base quantities are not defined in terms of other physical quantities. The base quantities are the minimum number of those physical quantities in terms of which other physical quantities can be defined. Typical example of base quantities are length, mass and time.
The measurement of a base quantity involves two steps: first, the choice of a standard, and second, the establishment of a procedure fro comparing the quantity to be measured with the standard so that a number and a unit are determined as the measure of that quantity.
An perfect standard has two principal characteristics: it is accessible plus it is invariable. These two requirements are often incompatible with a compromise has to be made between them.
For your information
order of magnitude of some distances