Relationship between Q and STC
brian.thomas at gsfc.nasa.gov
Mon May 10 12:18:57 PDT 2004
On Monday 10 May 2004 02:52 pm, David Berry wrote:
> > > 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.
That is not true. An axis is a phenomena that describes one dimension
of another phenomena. For example, "FLUX(Time)" [flux measurements
recorded for a particular time] is a 1-dimensional quantity where the
"TIME" quantity is both a phenomena (of "time") AND describes a
dimension of (and is thus an "axis") of another.
> 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?
It doesn't have to be providing the actual positions to be a Q. All that
is needed is that I pair a value with it (Frame information is *optional*,
and in the STC variant I provide, you cant put coordSystem information
on any of the STC components as that would be wrong).
Certainly, for the STC components there are enumerated values (as
I previously described) which are allowed to be there. Are there no phenomena
for which a limited, discrete set of values exist? I think so, and hence,
STC *can* and *is* a Q.
> > > 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
> > 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
Quit getting hung up on the values of the actual coordinates, thats a
red herring. The STC is a quantity that provides meta-data about coordinates,
the actual coordinate values are held by a parent Q.
> 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.
*sigh* I guess I will have to grab an example and put it in the email since
nobody is actually looking at the tarball. Please take a look and comment
on the example.
> 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
> United Kingdom | PR1 2HE
> OX11 0QX Tel. 01772 893733
> 01257 273192
* Dr. Brian Thomas
* Dept of Astronomy/University of Maryland-College Park
* Code 630.1/Goddard Space Flight Center-NASA
* fax: (301) 286-1775
* phone: (301) 286-6128 [GSFC]
(301) 405-2312 [UMD]
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