content, format, ctype, or xtype ?

[Mark Taylor]

On Tue, 12 May 2009, Paul Harrison wrote:

> 
> On 2009-05 -12, at 13:14, Mark Taylor wrote:
> 
> > 
> > I don't, any more than I think the fact that we require units as well
> > as utypes should be regarded as a defect.  What I'm talking about is
> > orthogonal to what utypes are for, which I take to be some kind of
> > semantic characterisation.  In fact, it's more like what you've
> > characterised as "lexical types" in your utype discussion
> > (http://nxg.me.uk/note/2009/utype-questions/):
> > 
> >  ... lexical types which indicate how a sequence of bytes is to
> >  be parsed (is 123 intended to be the string â??123â?? or the
> >  number 123 or possibly even a julian day number?)
> > 
> > Two columns could have the same utype (indicating an observation time)
> > but different [content/format/xtype/ctype/whatever-it's-called];
> > one could be supplied as an ISO-8601 string, and another as an MJD.
> > If I attempt your dereferencing trick and find out that the field
> > I'm looking at is "like a date", it doesn't get me very far with that
> > (even apart from the fact that I now need a network connection to
> > make sense of an otherwise self-contained data file).
> > Yes I can look at the datatype and have a guess, but this is messy,
> > and it still doesn't help me distinguish between JD and MJD.
> 
> 
> 
> Can this concept not be satisfied by the units metadata for a column? There is
> a "units" effort going on within IVOA at the moment - if this prescribed the
> format for certain specialized units like MJD, sexagesimal etc. then there
> would be no need to add a piece of "format" metadata

Using the units attribute would be a messy but pragmatic solution to
this that would do most of the work required.  Furthermore it is actually
used in this way in some existing VOTables, in contravention of the
VOTable standard (e.g. Vizier uses attribute values like unit='"h:m:s"').
It's inelegant though:

   1. MJD, sexagesimal, STC-S, ISO-8601 etc are not units, they're 
      something else.  However, in most or all cases, once you know 
      the representation information that I'm talking about here, 
      you don't need to specify units any more, so there is unlikely 
      to be a problem of where do the units go when this is filled in.

   2. VOTable specifies a specific syntax for the units attribute, see
      http://www.ivoa.net/Documents/WD/VOTable/VOTable-20080914.html#ToC27
      It is supposed to be parsable.  Clearly, using tokens like the
      above doesn't fit with this.  However, correct use of this syntax
      is very far from widespread.

I'd be happier with this solution than with the status quo
*if it was written into the standard rather than just used in an
ad hoc basis as now*.

Mark

-- 
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/