SYSML11 — SysML dimensions

Inconsistency among valuetype/unit/dimension

In Figure 8.4 p 43, the following multiplicities are given

A valutype may (0..1) have a unit and may have a (0..1) dimension

A unit may (0..1) have a dimension.

On page 49. there is a constraint

Constraints

[1]If a value is present for the unit attribute, the dimension attribute must be equal to the dimension property of the referenced unit.

This would mean that if the unit’s dimension is null, then the valutype’s dimension must also be null. This seems overly constraining.

In 8.4.2, it says

Because a unit already identifies the type of quantity, or dimension, that the unit measures, a value type only needs to identify the unit to identify the dimension as well.

This statement seems to say the unit must have a dimension, and the valuetype’s dimension can be null even though the unit’s is not. This statement therefore disagrees with the Figure 8.4 and the constraint on page 49

Discussion:
This issue is being deferred as part of the same overall discussion and
consideration noted under the resolution for Issue 12128, “8.3.2.9 Unit, 8.3.2.10
ValueType.” As noted there, the scope of any potential changes to the current
ValueType, Unit, and Dimension metamodel has ended up beyond the workload
that could be completed during this RTF. Relaxing the current constraints, as well
as extensions or changes to the current model, should continue to be evaluated
for future versions of SysML. In the meantime, a workaround could be to define
new units if necessary to carry different dimensions. The statement in Section 8.4.2 is in a usage example for the definition of value
types with SI units. It is not intended to imply that a unit must have a dimension,
just that if a unit does already identify a dimension, the dimension need not also
be specified. This language could be further clarified if needed in future updates
of the specification.
Disposition: See issue 12219 for disposition