content, format, ctype, or xtype ?

[Paul Harrison]

On 2009-05 -12, at 14:36, Mark Taylor wrote:

> 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.
>
I know that they are not SI units, and it is not pretty to a purist.

I think that these concepts are not as far away from units as first  
they might appear, because as you say once the "format" is known the  
"units" are pretty much automatically specified. Moreover, in saying   
"54963 MJD" it is just treating "MJD" as a non fundamental derived  
unit - and in this case it is very close to any reasonable definition  
of a unit.

The others are not units in the sense that using one of the scaling  
prefixes would be sensible - (and even 54.963 kMJD might be a little  
confusing ;-)) Sexagesimal is interesting in as much as degrees,  
minutes and seconds are separately outside, but acceptable to, SI -  
but it is clear that the compound use that is extremely common in  
astronomy does not follow the usual rules for formatting/processing of  
units.

>   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.

Given a suitable representation for the "format" aspect of then these  
rules could still be followed - e.g. some sort of escape characters  
e.g  "{h:m:s}" to indicate that the "unit" has special "format"  
requirements.

On the other pragmatic front, I suspect that it is mainly only time  
and position that have these shorthand lexical representations,  
however they have these because they are so common and deserve to be a  
special case (in units terms). Whilst these various concepts of utype,  
units and format are theoretically orthogonal, the space is so sparse  
that it might be better reduce the dimensionality by collapsing unit  
and format. In addition, it is well observed that the more metadata  
boxes that you expect people to fill, the less likely they are to put  
the right thing (or anything at all) in the right box.
>

Paul.