content, format, ctype, or xtype ?

[Arnold Rots]

A single Time Scale is not feasible, but I think it is reasonable to
restrict it to TT, UTC, and TDB (and their historic predecessors).
As far as time reference positions (I know, people want to keep this
out of the discussion, but just for reference), one could reasonably
restrict it to TOPOCENTER and GEOCENTER for the first two, and
BARYCENTER for TDB. Solar research will want to use HELIOCENTER, but
we need to find out from them what to pair it with.

  - Arnold

Francois Ochsenbein wrote:
> 
> (sent to 2 discussions groups dal + votable, since it concerns
> really both)
> 
> Thanks for the reactions about the VOTable/TAP connection;
> it just happened that Mark and me were thinking more or less
> simultaneously about enumerating the various possibilities
> for a final decision. I try here to give below some kind of 
> summary about the answers. Before just 2 comments about 
> 
> => Ray's suggestion of adding "anyAttribute" (possibility of
>    adding any extra attribute to FIELD): even though it's an
>    appreciable flexibility, I'm really afraid that would go
>    against our goal of interoperability
> 
> => Steve's remarks about time definition and leap seconds: 
>    personnally I would applaude if we would reach a consensus
>    about a SINGLE recommendation of time scale across the IVOA,
>    even if the recommendation would be to use something as 
>    strange as the number of seconds elapsed since a starting 
>    date (e.g. UTC 2000-01-01T12:00:00.0000 = JD2451545.000000000): 
>    at least we would be able (theoretically) to compare timestamps
>    unambiguously. The connection with database TIMESTAMP would not
>    be so complex, since SQL includes functions to convert a
>    timestamp into a number of seconds elapsed since some reference
>    date/time (which might be just the interval defined as a 
>    difference of TIMESTAMPs).
> 
>    
> Short summary of options:
> -------------------------
> 
> (a) Date-time, timestamp, ...
>     (a0) Restrict the expression of time to JD or MJD or ... number
>     (a1) Create a new attribute (which name ? which content?)
>          =M2 (Mark's option#2)
>     (a2) Create a new datatype (equivalent to 'char' as far as the
>              data storage is concerned, with an arraysize reflecting
> 	     the accuracy, i.e. '19' for full date YYYY-MM-DDThh:mm:ss)
>          =M1 (Mark's option#1) [
>     (a3) Use a utype related to the STC data model
>              (e.g. utype='stc:AstroCoords.Time.TimeInstant.ISOTime')
>          =M4 (Mark's option#4)
>     (a4) Use a UCD (e.g. ucd='time.iso8601'; could be secondary
>              e.g.  ucd='time.release;time.iso8601')
>     (a5) Use a special unit (e.g. unit='"iso8601"')
> 	 =M3 (Mark's option#3)
>     (a6) No, use STC-S string (e.g. 'Time TT GEOCENTER 2009-05-13T19:09:30')
>              [from Arnold] 
> 
> It seems difficult to derive a strong answer from the various
> messages posted, but the impression I have after reading your
> messages is that:
>   (a4) (ucd) is among the favorite solutions, provided it does 
>        not hurt the Semantics group; 
>   (a1) (new datatype) looks attractive [but conflict between
>         model and low-level data container?] 
>   (a5) could be accomodated (except Arnold?) [and after all, 
>        'iso8601' has something to deal 'time' unit].
>   (a1) strangely nobody seems to favor a new attribute...
> 
> Ah, Patrick, just about multiple utypes (or ucds): it's in fact
> possible to assign (indirectly) more than 1 utype (or ucd) to a
> field, via the GROUPs (a field may be referenced in several
> groups). A bit cumbersome, but on the other hand it can neatly
> underline multiple inheritences.
> 
> While writing this, Tom's message came in (placing (a5) at top);
> I would personnally basically agree with his argumentation.
> 
> --Francois
> =======================================================================
> 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
> =======================================================================
> 
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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/
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