SED Data Model: Questions and Comments

[David Berry]

Pedro,

> about the units handling, I think the units should be handled by the
> clients, not the Data Model.

I'm not sure what you mean by "handled" here. The DM should *describe*
the units, and the client shoudlinclude software for converting between
units.

The only information the client needs is
> the dimensions (equation) of the units together with the scale factor,
> therefore the units don't have to be stored in any specific format.

But we already have a standard form for specifying units given in FITS-WCS
paper one. What information does the dimensional analysis add which is not
already present in (or can be determined from) the units string? If it
adds nothing, then including a dimensional analysis in a data set would
seem not only redundant but also a little dangerous, since having the same
information in two different places in the DM introduces the possibility
of inconsistency. I know this wouldn't happen in a perfect world but...

So what can be done with a dimensional analysis which cannot be done
with a simple units string? There is publicly available software which
has unit mapping functions to parse these strings and automatically
determine the mapping between them (if any). And the dimensional analysis
is implicitly built into these units strings. For instance, to determine
if a given units string has the dimensions of flux per unit frequency, you
just use the unit mapping function to determine a mapping between the
given units and "W/m^2/Hz" - if a mapping is returned, then the supplied
units string has the dimensions of flux per unti frequency; if no mapping
is returned then it has some other dimensions. So you can then step
through the systems your code supports, looking for one which can be
determined from the given units.


> We have proposed to include those in the SSA protocol and they have been
> included in the SED data model. We have shown in several places how well
> this works for flux densities. The Quantity Data Model should include
> the dimensions as well.

I think this should only be done if the dimensional analysis adds any
information which is not already present in the units string. Does it?

> For other things like photometry, other information like the zero point
> and zero flux is needed, and I think we still lack a proper data model

What part would dimensional analysis play in such a data model? For
instance, how would it help in converting between antenna temperature and
flux density? Or indeed between some specified magnitude system and flux
density?

David