content, format, ctype, or xtype ?

[Arnold Rots]

Francois,

Here is the issue:
If a user queries a catalog for, say 90 < l <270 AND -30 < b < 30
you have to do an on-the-fly transformation of almost all your RA,Dec
entries in order to make sure you get the right subset.

With storage getting cheaper and cheaper it is far more cost-effective
to take the hit there and be able to perform the queries very efficiently.

The only ones you need are (FK4 or FK5 or ICRS) and Galactic and Ecliptic.

  - Arnold

Francois Ochsenbein wrote:
> 
> Hi Alberto,
> 
> Just a precision about vizier (at the end of your message):
> while coordinates in 'standard' frames are computed on the
> fly, they are not actually stored (currently) in vizier.
> I'm not a fan of adding gigabytes of computed data to gigabytes
> of original data; and as far as celestial positions are concerned,
> I would better promote a standardisation to a common reference frame 
> rather than asking every data provider to understand, compute
> and eventually store positions in galactic, ecliptic, icrf, fk4, 
> etc etc...
> 
> --Francois
> 
> >That would mean that ALL clients would have to know how to
> >interpret/translate/parse
> >ALL possible combination of units/ucds/utypes. I would not call this
> >"best and simplest"
> >from a client point of view.
> >
> >And if we consider the non negligeable fact, very well presented by
> >Fabien Chereau at the
> >last Interop in Baltimore, that coding a client that can make sense of
> >the different VOTables
> >returned by various data provides is a nightmare, then I think that the
> >solution to expose
> >to the VO external data as they are, it is actually (from a pragmatic
> >point of view)
> >a vo-stopper.
> >/* Fabien showed how Virgo handles SIA/SSA responses: a mixture of guesses
> >translated in complicated if/then/else playing with field names, ucds,
> >utypes, and units
> >and most of the time even that fails.
> >*/
> >
> >
> >If, on the other hand, the server already implements the translation of
> >a time field
> >internally stored as "number of seconds since 1-JAN-19xx", into the VO
> >standard (e.g.) iso8601,
> >all clients will know what to expect, without having to take any
> >assumption.
> >Only at the server side the correct knowledge to translate the internal
> >values
> >to the VO standard might be available. A client instead might take wrong
> >assumptions.
> >
> >Notice that I am only asking to standardise units and representations,
> >not reference frames,
> >and only for few well identified cases.
> >
> >So far we have spoken of:
> >
> >- angles (decimal degrees) [RA is an angle]
> >- timestamps/datetimes (iso8601?)
> >
> >and I would add
> >
> >- time intervals (like exptime, period; in seconds?),
> >- wavelengths (meters?),
> >- frequencies (Ghz?),
> >- energies (keV?).
> >- magnitudes (in mag, and not in eg dmag)
> >
> >more?
> >
> >It should not be a big deal to standardise those most used concepts. The
> >rest untouched.
> >
> >Alberto
> >I appreciate the problem of exposing the original data as they were
> >expressed by the authors of a catalog.
> >That is indeed important, and I am even thinking whether we should pass
> >the original
> >data untouched (as a display-string attribute?) along with the standard
> >representation of it.
> >Again, we are only talking of few quantities here...
> >Vizier already took this kind of approach when it was decided that the
> >original catalogs are stored
> >and served as they were conceived by the authors, but extra columns
> >(e.g. RA,DEC FK5 J2000)
> >are computed, stored, and served to allow homogeneous access throughout
> >the entire collection
> >(the MAIN ucd was invented, I think, for this).
> =======================================================================
> Francois Ochsenbein    ------   Observatoire Astronomique de Strasbourg
>    11, rue de l'Universite 67000 STRASBOURG  Phone: +33-(0)390 24 24 29
> Email: francois at astro.u-strasbg.fr (France)    Fax: +33-(0)390 24 24 17
> =======================================================================
> 
--------------------------------------------------------------------------
Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------