STC in VOTable: Time utypes

[Arnold Rots]

I will resist the temptation to start a diatribe on utypes ;-)
The problem is that utypes provide the leaves of the structure in
isolation and this is particularly uncomfortable when it comes to
substitution groups.
I can't remember whether this was brought up before, but one solution
might be to handle substitution groups explicitly in this way:
use the head element in the utype and add the choice from the
substitution group separately. Adding it as an attribute would be the
intuitive solution, but that would get complicated when there are
multiple layers of substitution groups. A better solution might be to
put it in parentheses:

      stc:AstroCoords.Time.TimeInstant.AbsoluteTime(ISOTime)

That also gets us out of the issues with spatial frames:

     stc:AstroCoordSystem.SpaceFrame.SpaceRefFrame(ICRS)

Cheers,

  - Arnold

Markus Demleitner wrote:
> Dear VOTable and DM folks,
> 
> On the issue of a common container I've received only positive
> feedback, so I'll go ahead and have the next draft include that.
> 
> Meanwhile, another issue has come up that I'd like to hear opinions
> on: The time utypes.  Currently, they look like this:
> 
> stc:AstroCoords.Time.TimeInstant.ISOTime or
> stc:AstroCoords.Time.TimeInstant.JDTime.
> 
> I've just put the following diatribe on this on the wiki page
> (http://www.ivoa.net/cgi-bin/twiki/bin/view/IVOA/STCInVOTable):
> 
> """
> That is bad for at least three reasons:
> 
>    1. clients have a hard time figuring out what column a time or a time 
>       error or whatever is (since they need to check at least three utypes)
>    2. we have xtype in VOTable for this purpose, and having both utype 
>       and xtype specify something then requires some rules on what to 
>       do when the attribute contradict or if there is some inference 
>       between them
>    3. data models should IMHO not be concernded with serializations
> 
> So -- what's to be done?
> 
>    1. Keep everything as is (ugly, but probably workable; declare 
>       clashes between utype and xtype as undefined)
>    2. Elide ISO/JD/MJDTime by special rule (but that's yet another 
>       special rule, and you cannot build an STC-X tree from utypes any more)
>    3. Change STC-X (would probably take a while and might be used 
>       to clean up some other ugly spots we have here, but it's a
>       major undertaking that would seriously delay STC-in-VOTable)
>    4. ???
> """
> 
> (at the very bottom).  Emboldend by the feedback to my last question,
> I thought I'd ask around again for opinions and suggestions --
> preferably on the wiki.
> 
> Thanks,
> 
>           Markus
> 
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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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