[TAP] data type for column metadata

[Arnold Rots]

Patrick Dowler wrote:
[ Charset ISO-8859-1 unsupported, converting... ]
> On 2009-03-25 08:13:26 Markus Demleitner wrote:
> ...
> 
> > So -- I'll support basically any choice of SQL types being part of
> > TAP metadata; I think we definitely should have timestamps in there
> > (though of course, people are free to store times in floats, integers
> > or varchars if that's appropriate; it's just that there is one
> > standard way to store and transfer timestamps, and everything else
> > has to be manually supported).
> 
> I take this as a proposal for one allowed iso8601 timestamp variant (mentioned 
> earlier, without timezone) and a requirement that all timestamps are in UTC. 
> Correct? 
> 
> Comments? Votes?

No:
All non-astronomical time stamps (like modification times) should
be in UTC.
But astronomical time stamps (i.e., time stamps associated with the
contents of the data) should be allowed to be in other Time Scales (I
repeat the list: TT, (ET), UTC, TCG, TDB, TCB, TAI, GPS); and a
reference position is also needed for correct interpretation.

I am not saying that all services need to be able to handle all time
scales (actually, that is impossible), but the information needs to be
there. For instance, if a database contains bary-centered TDB, it is
not practical (if not impossible) to deliver geocentered UTC; but one
should know the difference.

To return to the data types for communicating time information:
- Restricted ISO-8601
- Some form of floats for JD, MJD (unit='d' implied)
- Some form of floats for elapsed (relative) time since a given zero point

I am saying "some float": note that the JPL ephemerides handles time
in JD(TT) as a vector of two doubles; one contains the integer part,
the other the fractional part. This is to say: a double may not always
be good enough.
And it is the reason why some datasets use the relative time stamps.

  - Arnold

> 
> ...
> 
> -- 
> 
> Patrick Dowler
> Tel/T?l: (250) 363-0044
> Canadian Astronomy Data Centre
> National Research Council Canada
> 5071 West Saanich Road
> Victoria, BC V9E 2M7
> 
> Centre canadien de donnees astronomiques
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> 
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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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