SYSML13 — Improvements to QUDV for SysML v1.3

There are mistakes and inconveniences in the model and model library of quantities, units, dimensions and values (QUDV) in SysML v1.2 as defined in Annex C. Furthermore the ordering of the sections in Annex C can be made more logical to improve the readability.

Remove Quantity from Annex C.5. The reason is that the concept of quantity is
fully implemented by a value property typed by a ValueType. Therefore an
additional class/block Quantity is superfluous and confusing.
The QUDV model can be simplified by merging DimensionFactor and
QuantityKindFactor into one concept QuantityKindFactor.
Remove property definition for symbolicExpression, since this is only an
informational (presentation) property that can be derived from the ordered set of
factor : DimensionFactor.
In SysML 1.2, Figure C.8, a SystemOfUnits had a many-to-many relationship
with SystemOfQuantities. This precluded aligning SysML 1.2 QUDV tightly with
ISO 80000-1:2009 where the notion of ‘system of units’ (ISO 80000-1:2009, 3.13)
is defined for a single ‘system of quantities’ (ISO 80000-1:2009, 3.3).
Furthermore, Section C.5.2.22 Unit did not define the property Unit::quantityKind
: QuantityKind[0..1] shown in Figure C.8 even though this property is extensively
used for the specification of coherence analysis in section C.5.2.21
SystemOfUnits, Constraints [1,2].
The meaning of the undocumented association between Unit and QuantityKind
should be revised to support the capability specified in ISO/IEC Guide 99:2007
about a coherent system of units (item 1.14), Note 1:
A system of units can be coherent only with respect to a system of
quantities and the adopted base units. The adoption of a Unit as the base measurement unit for a given QuantityKind is
understood to be a distinguished subset among the set of all QuantityKinds that
are the measurementUnits for a particular Unit. A Unit can be the measurement
unit of many QuantityKinds and a given QuantityKind can have many
measurement Units per Note 2 of ISO 80000-1:2009, 3.9:
NOTE 2 Measurement units of quantities of the same quantity dimension
may be designated by the same name and symbol even when the
quantities are not of the same kind. For example, joule per kelvin and J/K
are respectively the name and symbol of both a measurement unit of heat
capacity and a measurement unit of entropy, which are generally not
considered to be quantities of the same kind. However, in some cases
special measurement unit names are restricted to be used with quantities
of specific kind only. For example, the measurement unit ‘second to the
power minus one’ (1/s) is called hertz (Hz) when used for frequencies and
becquerel (Bq) when used for activities of radionuclides. As another
example, the joule (J) is used as a unit of energy, but never as a unit of
moment of force, i.e. the newton metre (N · m).
The above makes sense in the context of a particular relationship between a
SystemOfUnits and a SystemOfQuantities. It would be more difficult to express
the above in a more generalized context like that of SysML1.2 with many-tomany
possible relationships between SystemOfUnits and SystemOfQuantities.
Consequently, the QUDV model is revised to focus on the most common case in
practice where a SystemOfUnits is defined for up to a single SystemOfQuantities.
The reduced generality of the QUDV model is intended to simplify the use of
QUDV in practice and is explicitly conveyed via new invariants (see below for
new constraints added for C.5.2.20 and C.5.2.21).
In SysML 1.2, Figure C.10 is not consistent with the text which includes OCL for
deriving the non-derived property SystemOfQuantities::dimension :
Dimension[0..*] (see C.5.2.20 SystemOfQuantities, Constraint [1])
Since SysML 1.2 was published, the major parts of ISO/IEC 80000 have been
published with definitions for the International System of Quantities (ISQ) and the
International System of Units (SI). Annex C.4 is revised to be based on this
authoritative source rather than the NIST special publication it formerly
referenced. The SysML QuantityKind and Unit definitions in Annex C.4 are
revised to follow the naming and organization of the quantity and unit definitions
from Part 1 of ISO 80000, which cover the same base and derived units as
before. Unit symbols have also been added to the SysML Unit definitions.
The text in Annex C.4 is revised to indicate how the definitionURI strings of its
QuantityKind and Unit definitions could be used to refer to corresponding QUDV
definitions in a separately published model library. Figures C.10 and C.11 in
Section 5.4.1 continue to provide examples of such QUDV definitions that could
be expanded to the full set of quantities and units from ISO 80000-1.
Specific definitionURI string values for reference to QUDV definitions have not
been included in Annex C.4, not only because they would clutter the diagrams,
but because they could be automatically supplied as part of the publication of
more complete, machine-readable versions of both model libraries.
Because Annex C.4 continues to define a model library consisting only of SysML
QuantityKind and Unit definitions, without any direct dependence on QUDV or
other QUDV model libraries, there is no inherent problem with the ordering or
logical dependence of the current Annexes. No change is made to the current
Annex C sections other than a new name for the ISO 80000-1 model library in
Section C.4.