SED Data Model: Questions and Comments

[Pedro Osuna]

Dave and all,

maybe we should discuss this off-line, but as it has jumped off here,
I'll keep on...

[...]FITS WCS paper one suggests that unit strings should be
standardised[...]

yes, and again the problem is that some data providers do already have
their units written in other formats. Some of them are inside very old
"standard" names inside FITS files that will never be changed, just to
give an example.

[...]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?[...]

because the dimensional analysis standard consists of only one line,
whereas the units standard consists of many names. And I'm not asking
for removal of the string names, I'm asking for inclusion of dimensional
parameters. 

For you interest, I was asked by the FITS community to send information
about this dimensional analysis thing, and I attach the answer back from
Greisen himself (one of the writers of the FITS WCS III paper). He
understands that the idea is nice and puts his reasons to not include it
in paper III (as he understood that was the proposal, which was
certainly not). Among them is the absence of rigorous formulation, and
that's the reason why we are writing something on it. Please see the
attached mail.

[...]But is also introduces extra redundant meta-data, increasing data
size and complexity[...]

as I say, the dimensional parameters are just two, normally the same for
many of the providers' data. Not much overhead.

[...]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)[...]

we can help people on this. On the other hand, data providers will not
have to change their units inside their files, but just give the correct
dimeq-scaleq in the metadata. This would allow their data (though old as
they might be) to be able to play in the VO without having to modify
them. Still, I believe it's worth the effort.

Cheers,
P.


On Wed, 2005-02-16 at 12:51, David Berry wrote:
> 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
-- 
Pedro Osuna Alcalaya

 
Software Engineer
Science Archive Team
European Space Astronomy Centre
(ESAC/ESA)
e-mail: Pedro.Osuna at esa.int
Tel + 34 91 8131314
---------------------------------                                                                                
European Space Astronomy Centre
European Space Agency
P.O. Box 50727
E-28080 Villafranca del Castillo
MADRID - SPAIN
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