content, format, ctype, or xtype ?

[Mark Taylor]

On Tue, 12 May 2009, Markus Demleitner wrote:

> > - Or are we sidestepping a responsibility to choose appropriate defaults?  
> > Units can be forced into narrow options, e.g., RA can be required to be 
> > either the decimal degrees or sexagesimal hours that correspond to 99.9% 
> Well, you'd have my vote for a canoncial representation of datetimes
> any minute, and coordinates as well.  But I doubt you'll get all the
> interested parties agree just *what* the canonical representation
> would be.
> 
> Plus: How would you tell your two representations apart ("if it's a
> string, it's sexagesimal, if it's numeric, it's decimal degrees" has
> an eerie air about it...)

This is of course what application authors such as myself currently do.
One problem is that it is, to put it politely, "eerie" as Markus says.
Another is that you need to know that the thing is an angle of some
sort before you try doing something like that.  This requires 
reasonably complicated, and perhaps impossible, analysis of UCDs
(need to check separately whether it's one of pos.az.*, pos.bodyrc.*,
pos.earth.*, pos.ecliptic.*, pos.eq.*, pos.galactic.*, 
pos.supergalactic.* - is that the lot? - and know for each whether
they are customarily done in hms or dms, maybe it's some spherical
coordinate system not yet in the UCD1+ list?) and of utypes
(do I know all the data models that might have an angle in them?
can I match the utype strings appropriately, with correct use of
namespaces and so on?  or should I use Norman's idea of interrogating
remote knowledge systems about what utypes resemble what other
utypes?  Does a data model even exist for the angle that I'm talking
about?)  For a big complicated VO application, or one that has a good
idea of where its information is coming from (e.g. SSA, which has
mandatory utypes) that may be OK.  But for a lightweight tool that 
wants to be able to recognise sexagesimal strings, or iso-8601 times, 
or MJDs when it sees them, it's really a lot of work.

-- 
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-928-8776 http://www.star.bris.ac.uk/~mbt/