Grapsing vaguely at what you are describing, I still can't see how
units differs from other metadata, either theoretically or practically.
Jumping to the summary bit of your message:
> The link to units (getting back to that, David) is that when
> assembling and using this information, I really couldn't see a place
> for the units information which is in the VODataService element
> above. The statement "USNO-B's 'ra' column is a type of pos.eq.ra"
> is true independently of units. Once I've established just what this
> USNO-B column is supposed to be (aha, an RA!), then I'm going to have
> to discover what units the data there has, in order to actually read
What about all the other metadata needed to interpret the RA value?
Things like, reference system (FK4, FK5, ICRS, etc), equinox, epoch...
Without these things, it's still not possible to convert between two
general RA values (indeed, even *with* these values, it still would not be
possible unless you also had the associated Declination value).
The amount of metadata required is even larger when trying to convert
between spectral values (e.g. velocities). You then have to know the
reference position on the sky, the rest frame, the rest frequency, the
definition of velocity being used, etc.
The units is just one more item of metadata needed in order to be able to
interpret the column values. To do anything of any practical value with
the columns, you need to know values for all the required metadata, of
which the units is just one item.