SED Data Model: Questions and Comments

[David Berry]

Pedro,

> parsing of strings is the traditional way to handle units, and we
> believe there are examples more than enough of cases where units are
> named wrongly, despite any effort to homogeneise unit names (which vary,
> by the way, sometimes from FITS WCS paper I to A&A recommended units
> conventions (a la Vizier, I believe), to CODATA ones, etc.).

Sure, people need to abide by some standard language if communication is
to be possible. FITS WCS paper one suggests that unit strings should be
standardised, and you suggest that dimensional analysis description should
be standardised. Either way, data provides have to check that their data
conforms with *something*. So, given that some standardisation effort
is necessary, and that data will presumably always include a human
readable units string, why not standardise that string rather than
introducing an additional dimensional analysis standard?

> However, we insist that for superimposition of different
> spectra in different units, the dimensional approach gives -even
> algorithmically- a lot of benefits.

But is also introduces extra redundant meta-data, increasing data size and
complexity, gives rise to the possibility of inconsistency within the
meta-data, and requires more effort on the part of data providers (in
that they have to work out what the dimensional analysis and scale
factor are).

David