A plehtora of Quantities

[Guy Rixon]

On Fri, 14 May 2004, David Berry wrote:

> Guy,
>
> > An SED is a list of flux-frequency-bandwidth tuples, where each element of
> > each tuple is a quantity (i.e. has value, unit, accuracy).  No way is each
> > point as a whole "a quantity" in normal scientific usage.
>
> You're right of course for normal scientific usage, but one *could* in
> principle define such a quantity, if there were benefits in doing so??

Yes, but not if we want it to be truly intuititive. maybe that's not so
important.

> > Furthermore, in a
> > composite SED, each quantity of each point can have _different_ units. I.e.,
> > there's nothing to share between adjacent quantities; therefore, no motivation
> > to make internal parts of a quantity list-valued.
>
> Ah yes - if you have an SED like this then you are right of course. May be
> we would be better talking about a spectrum rather than an SED. For an
> SED in which each sample differed in units you would indeed need a list of
> Quantities - that's certainly allowed.

So far in astronomy I've seen composite SEDs as above (e.g. the AVO
demonstrations of 2002) and spectra that were uniform enough to store as 1D
rasters. The two kinds are rather separate.  That suggests to me that the
models could be separate too.

> > This list-valued thing is what breaks Q for me.  I would like to see the Q's
> > inside the list, not the list inside the Q. _That's_ intuitive, and it also
> > means the listing structure can express the relationship between the Q's; the
> > standard Q structures don't have to deal with relationship.
> >
> > Is the real reason for the list values inside the Q is to save space in the
> > XML representation?
>
> Not really- I think the idea is that the "list inside the Quantity" more
> closely reflects the general case where you have a whole list of *values*
> for a single (possibly compound) *phenomenon*. So a Quantity contains a
> "list of values" (the ValuesList of Mapping) and a description of the
> single phenomenon (the Frame).

OK, I understand.  But if we're modelling Phenomemon, then that's a different
and much bigger thing than Quantity.  Can we have a nice, simple Quanitity as
a separate product?


> > I don't buy this.  If we need to save space then we
> > shouldn't be using XML.  Or is it to try and map a Q to things like image
> > rasters? In that case, the system is compromised from the start and it
> > would be better to have different, cleaner structures for the XML case.
>
> Allowing a Q to represent an N-d array of homogenous values (such as are
> used to store spectral cubes, images, spectra, etc) has indeed been a
> major influence in the design. This is largely because such structures
> are obviously needed and represent a more general case than a single
> value. If you have many values for a single phenomenon, why not model it
> like that?

Sure, such structures exist and are an important special case.  IMHO, the case
is important enough to be handled separately from scalar quantity.


>
> David
>

Guy Rixon 				        gtr at ast.cam.ac.uk
Institute of Astronomy   	                Tel: +44-1223-337542
Madingley Road, Cambridge, UK, CB3 0HA		Fax: +44-1223-337523