Relationship between Q and STC
brian.thomas at gsfc.nasa.gov
Mon May 10 13:50:32 PDT 2004
On Monday 10 May 2004 04:29 pm, David Berry wrote:
> > > 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.
> The point I was making that an axis *describes* a phenomenon but does not
> provide any values for that phenomenon. So:
That is not true either. I may choose to store explicitly the values
of the axis OR I may choose to use a mapping. This is an old topic
I had thought we were past...
> 1) "topcentric radio velocity in units of km/s" *is* an axis and is
> equivalent to a 1D Frame. This is "the phenomenon".
> 2) "23.345 34.23 100.23" are values and can be stored in a ValuesList or
> generated by a Mapping.
> 3) Put together the Frame and the Mapping (or ValuesList) and you have a
> Quantity, That is, a a list of values for some phenomenon.
> This is why I say an axis by itself (without any associated axis *values*)
> is not a Quantity, it's a Frame.
Disagree. An axis is a quantity which either is deriving its values from
a mapping or from explicit storage. You can always down cast the axis
to "Frame" interface if you like, but its not needed in the serialization.
In fact, its more compact and looks better as a quantity with values in place
(axis that is).
> > 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.
> This comes back to what a Quantity is. The above says that STC *is a*
> Quantity, but it has no values (the values are held elsewhere, in a
> "parent" Quantity).
The "values" of the STC are other quantities which appear as "members".
> So we come back to the same point, is the
> purpose of a Quantity to hold *measured values* for some
> phenomenon. Presumably you would say not since you say that STC is an
> example of a Quantity which does not contain any values.
Can not a phenomena be described in terms of constituent phenomena?
This is all that is happening with STC/Q proposal. I don't believe that
all phenomena *must* be derived from measured values. What about
constants? Or theoretical data that I might generate? Do they not describe
some physical phenomena as well? And since when must the Q describe
*only* physical phenomena (??). What if I want to pass you a machine-readable
list of numbers? The Q works well for that purpose as well..
> > > 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.
> Apologies! Your example made me realise that I have not been very precise
> about what I mean by "STC". I have been equating "STC" with the
> "CoordSystem" element. So in my previous points, make a global edit of
> "STC"->"CoordSystem". Obviously the STC schema also includes the "Coords"
> element which *does* include axis values, and so *is* equivalent to a
Ah, so are we arguing then?!?
= confused in DC on this last point.
> 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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