KERML — Occurrences do not identify local spatial frame

Occurrence::localClock:Clock identifies a "time reference for this Occurrence" (9.2.4.2.13 Occurrence), but no feature is defined for a local spatial frame to identify a spatial reference for it (i.e., occurrences refer to a clock to measure time, but not a measuring rod for space).

No change

The localClock feature was added to Occurrence specifically so it could be used as a default for time triggers. The localClock has to be in Occurrence, because it has to be propagated down to both contained Structures and contained Performancs. There was no such driver for spatial frames. In addition, SpatialFrame also depends on Clocks and uses localClock for defaults, while there is no such dependence of Clock on SpatialFrame.

Further, adding a feature for a "spatial reference" to Occurrence would not be consistent with the current model SpatialItem in the SysML Geometry Library, which is the primary expected application of SpatialFrames. And a SpatialItem actually is a SpatialFrame (as well as an Item). In this model, the "spatial reference" is more properly the CoordinateFrame, which is not a SpatialFrame, but, rather, a VectorMeasurementReference. And VectorMeasurementReference is defined in the Quantities and Units library, and, so, not available in KerML.

(See also KERML-282 and KERML-283 for more general statements of the issues without proposing solutions.)