A plehtora of Quantities

[David Berry]

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??

> 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.

> 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).

> 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?


David