Relationship between Q and STC

[David Berry]

Brian,

> > Oooh. Not sure about that! STC is just *directly* equivalent to a Frame
> > (excepting the Region issue).
>
> What about the axes information? Why not re-use the Q for this?

Because an axis is not a Quantity, it's a phenomenon. A Quantity is for
recording values of some phenomenon and so encapsulates two components:

1) the Frame which identifies and describes the phenomenon
2) the (Mapping or ValuesList) which provides the values of the phenomenon

An STC document does not specify any particular *positions* in space-time
coordinates, it just describes a coodinate system, so what values are
you going to put into an STC Quantity?

> > But a Quantity should be "a rule for obtaining values", and STC
> > specifically excludes issues to do with obtaining values - its just
> > defines the coordinate syetem and so is a Frame, not a Q.
>
> Thats not true. I see components of the STC as quantities,
> for example "StandardReferencePosition" is a quantity, albeit one
> which is allowed to  take one of a restricted set of values, e.g.
> "GEOGENTER", "BARYCENTER", etc.

"StandardReferencePosition" simply corresponds to a property of the Frame
described by the STC document. There is no need to consider it to be a
Quantity.

> And if I can get all the component parts of the ST to be Q's (and
> I can) then this means at the STC itself can be a Q (as StandardQ
> allows holding "member" Q's).

But again, the whole notion of an STC document is that it describes a
coordinate system without specifying any positions within that coordinate
system. Your star catalogue has columns corresponding to RA and Dec, the
associated STC document describes this (RA,Dec) system but does not itself
include the numerical values stored in the table columns. The STC document
is the Frame, the table columns are the ValuesLists, and both *together*
form a Quantity.



David


----------------------------------------------------------------------
Dr David S. Berry    (dsb at ast.man.ac.uk)

STARLINK project		 |	Centre for Astrophysics
(http://www.starlink.ac.uk/)	 |	University of Central Lancashire
Rutherford Appleton Laboratory	 |	PRESTON
DIDCOT				 |	United Kingdom
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