Units needed in ADQL-0.7

[Thomas McGlynn]

Clive Page wrote:

...

> (a) the physical units for each column must be standardised so that
> everyone in the community knows that galactic latitude must be in
> degrees and any data centre using anything different has a mandatory
> permanent conversion job to do.
> 
> or
> (b) we need a way of specifying units in the string (and in the XML).
> 
> I think that (a) is too difficult, as there are just too many different
> columns and possible units, and argument as to what should be the
> "standard" would never cease.  Using (b) we need a notation for units.
> 
> The nearest thing to a standard notation that has so far been proposed is
> in section 3.2 of this document from CDS:
> http://vizier.u-strasbg.fr/doc/catstd.htx
> I think this looks a pretty good solution to the problem.
...
There is a standard string  format for units defined in the
second FITS WCS paper (e.g., http://www.edpsciences.org/articles/aa/pdf/2002/45/aah3859.pdf).
Since this is nominally (or will soon be) an accepted astronomical standard
that might be a better starting point for standard units string.

However, I'm not convinced that standardized units might not
be feasible.   If we are able to convert units, why
not convert them as we are sending queries onto the wire.  So what
flows over the wire is always in standard units.  Otherwisee
every site is faced with supporting transformations between every possible pair
of compatible units rather than just between standard and local units.
I think it makes the problem easier for everyone if we standardize units over
the wire.

    query/local units for querier ->
         query/standard units ->
            query/local units for database
    response/local units for database ->
          response/standard units ->
             response/local units for querier

While there are a lot of units used, I don't know that it would be too difficult
to build a rule that gave the standard form of a unit given some non-standard
form.  Getting people to agree on one rule might be harder!

	Tom