[QUANTITY] Pulling it all together

[DIDELON Pierre]

Hi all,

just few comments which, I hope, are relevant and not too redundant.

> Date: Thu, 22 May 2003 00:35:39 -0500 (CDT)
> From: Ray Plante <rplante at poplar.ncsa.uiuc.edu>
> To: dm at ivoa.net
> Subject: [QUANTITY] Pulling it all together
> 
>   1.  Use.   
> 
>       Important uses of this model:
>         *  building a metadata schema (in XML)
> 	*  creating software interfaces

As far as I understand the VO processus, we don't try to model
every thing in astrophysics, but we try to furnish a general 
framework to ease data exchange (and more if possible).
So, the first goal seems to be a clear (and as simple as possible) 
definition of metadata.

> 
>   2.  Simplicity vs. Complexity.  
> 

Simple things must stay simple, introducing complexicity 
should not imply a complex behaviour for every thing;
inheritence, sub-classing and composition seems a god way.

> 
>   3.  Quantity as a Building Block
> 
>       An important motivation for this model is to allow its re-use in
>       creating more complicated models.  Some ramifications are:
> 
>        *  Different quantities will be used in combination.  We want
>           to avoid redundancy of information.  
>        *  It's appropriate that we think about and describe how a
>           quantity could/should be used in the for describing more
>           complex things.

I like the groups as mentionned by Roy.
An argument of re-usability would be to try to re-use
general element of VOTable which fullfill our needs,
as for axample <PARAM><INFO><GROUP>...
Is this unrealistic, not politically correct,
or only relevant for the next step... implementation?

> 
>   4.  Where to put things: Inside Quantity vs. Beside it.
> 
As you can guess from above I would prefer the composition
of simple elements, which seems the best way to keep 
simple things simple.


Concerning the reference frame, it is quiet obvious that sometimes,
when basic default assumption are not straighforward, this information is
crucial. As UCD try to catch the essential meaning of data (semantic?)
to be able to handle it, some of them are certainly related to this subject
(of Coor.ref.frame). Does somebody have an idea about it, and give some 
insight of what is available? It is not clear to me, it exists certainly,
but it is perhaps burry inside the UCD complexity (and fixed name).
May be the refurbishing of UCD will clarify that with some
common suffix and/or prefix or grouping protocol?

Concerning error, I feel that it is, after the "value", the second parameter
of great importance when working with data. 
Whatever you want to do, you will certainly need it, even with no much
additional information, like for example for comparing data (by chisquare), 
plotting (with error bars) or crossidentifying.
The question is; are data without error realy useful? for what?

> 
>   5.  Scalars and Vectors.  
> 
how did you forsee vector def. length+elems, elems+terminator or both?
> 
> 
cheers,
Pierre