Improved categorisation of derived units

[jesper-friis]

Add MeasurementUnitByBasis

DerivedUnit is currently a subclass of NonPrefixedUnit. However, the following statement from the SI-brochure

Historically, SI units have been presented in terms of a set of – most recently seven – base
units. All other units, described as derived units, are constructed as products of powers of
the base units.

seems to state that MeasurementUnit is a disjoint union of BaseUnit and DerivedUnit, although it is little unclear whether derived units may also include prefixes. However, that is clarified from the following example in the definition of derived unit in VIM:

EXAMPLES The metre per second, symbol m/s, and the
centimetre per second, symbol cm/s, are derived units of
speed in the SI.

Hence, in OWL RL, BaseUnit and DerivedUnit should be subclasses of MeasurementUnit and disjoint.

Suggestion:

• Add a MeasurementUnitByBasis with disjoint subclasses BaseUnit and DerivedUnit.

The consequence of making BaseUnit and DerivedUnit disjoint should be checked.

Replace MeasurementUnitByPrefix with MeasurementUnitByCoherency

The value of introducing PrefixedUnit / NonPrefixedUnit is questionable. First, the concept prefixed unit is neither defined in VIM nor SI-brouchure. Second, it is unintuitive, since a unit like cm/s is a NonPrefixedUnit according to the definition of PrefixedUnit.

The categorisation of coherent / non-coherent units is much more useful. They are also defined in VIM and the SI-brochure.

Suggestion:

• Add MeasurementUnitByCoherency with disjoint subclasses CoherentUnit and NonCoherentUnit.
• (Optionally) remove MeasurementUnitByPrefix.

Remove SINonCoherentDerivedUnit

All SI non-coherent units are also derived units. Hence, the class SINonCoherentDerivedUnit provide nothing new.

Suggestion:

• Make SINonCoherentUnit a subclass of SIDerivedInit
• Remove SINonCoherentDerivedUnit (changing all its subclasses to be subclasses of SINonCoherentUnit)