SED Data Model: Questions and Comments

[David Berry]

Pat,

> I think the big win for dimensional analysis is that when you write code to
> handle unit strings, you essentially parse a bunch of ugly stuff using lookup
> tables and trun it into a form you can work with  - the dimensional analysis
> exponents and scale factors - then you use that to convetrt values to your
> preferred unit system.

I'm not sure it's a good principle to duplicate information in different
forms within a DM just to make it easier to write an interpreter. For
instance, FITS-WCS paper II expressely says this shouldn't be done in
the context of FITS WCS headers. The unit string has all the information
you need, in a compact human readable form. Seems good to me.

> So why bother having everyone write that parsing code, which everyone knows
> seems simple enough when you have m <-> ft and maybe sec <-> days but quickly
> degenerates into something very fragile that you keep tacking extra bits
> onto, keep adding rules to your lookup tables, etc.

It shouldn't be necessary or desirable for everyone to write their own
code, since code sharing is presumably what we are about.

> As for converting existing data, it isn't all that bad because only the data
> provider has to understand their own small set of unit strings they used.
> That could be done without a general purpose parsing toolkit/code...

What ever arguments exist apply equally to both schemes. Either you
modify your existing data to correct non-standard unit strings, or you
modify your data to include a dimensional analysis. Except of course
interpreters could be written to be tolerant of the most common
non-standard forms of unit string, meaning that data providers would only
need to correct non-standard unit strings if they were *really* whacky or
ambiguous.

> So, I would go for this in principle, despite having not read the paper :-)

I must confess, I am not completely sure how it works. For instance,
if you had two unit strings "Jy" and "Jy/sr" which have the same
dimensionals (since solid angle is dimensionless), how does the
dimensional analysis approach realise that they are not the same unit?
Pedro?

David