[QUANTITY] Plea for pragmatism

[DIDELON Pierre]

> From dsb at ast.man.ac.uk Wed Oct 29 14:43:28 2003
> Date: Wed, 29 Oct 2003 13:43:25 +0000 (GMT)
> From: David Berry <dsb at ast.man.ac.uk>
> To: DIDELON Pierre <dide at discovery.saclay.cea.fr>
> cc: Alberto.Micol at eso.org, dm at ivoa.net
> Subject: Re: [QUANTITY] Plea for pragmatism
[snip]
> 
> > > Having said that, I think it would be useful to have a simple overview of
> > > what a candidate data container may look like, just so that we can check
> > > if it is able to meet the requirements which emerge as we develope the
> > > OBSERVATION model, and change it as needed. I'm thinking here of something
> > > as simple as the following:
> > >
> >
> > Once we have to tackle this subject, I think I would agree with most
> > of the points below, except QUALITY, and add COVERAGE, COMPLETNESS
> > (lacunarity) at least.
> 
> I was thinking of COVERAGE and COMPLETENESS as a components of the
> OBSERVATION rather than the QUANTITY. You may want to use a QUANTITY to
> define the coverage. For instance, the spectral coverage of an observation
> could be specified by a 1D QUANTITY holding a bandpass.

Yes, me too I was refeering to OBSERVATION when talking about COVERAGE and so on.
That's why QUANTITY can be used to define COVERAGE etc, that it has to be kept
very simple, even if too simplistic.
Agreed.

> 
> > > QUALITY - gives a set of flags and/or enumerated values for each value in
> > > the DATA component.
> >
> > This is a very complex information, very diffucult to attribute, which can
> > evolve with time, depend on the subject studied...
> > One data set unusable for a certain study, can be perfect (if not better)
> > for another one.
> 
> Again, this is not suppose to characterise the overall usefullnes of the
> observation for some purpose, but just simply to assign specified flags to
> individual data values (e.g. "this value is saturated", "this value
> is vignetted", etc ). Characterising the usefullness of the whole
> observation would not be the task of a QUANTITY.

I agree too, it refers to OBSERVATION.

> 
> 
> > > WCS - Contains a collection of world coordinate systems in which
> > > positions within the DATA component can be described, together with
> > > Mappings which describe how to transform positions between different world
> > > coordinate systems.
> > >
> >
> > Reference frame is more general and certainly more appropriate?
> 
> The idea is that the position of a data value can generally be described
> in terms of several different coordinate systems, not just one "reference
> frame". For instance, for a CCD image of the sky, a position may be given
> in pixel coordinates, or focal plane coordinates, or sky coordinates. The
> WCS component would contain descriptions of all of these coordinate
> systems, together with Mappings which allow a position to be transformed
> between any two coordinate systems. This could be done in a "toolkit"
> manner to allow the system to be extended easily to cover data (e.g. solar
> or STP data) which is more complex than a simple CCD image of the sky.
> See:
> 
> http://www.ivoa.net/internal/IVOA/InterOpMay2003DataModel/Structuring-WCS.ppt
> (or, if you object to powerpoint,
> http://axp0.ast.man.ac.uk/~dsb/ast/IVOA_WCS_talk.html)

You are certainly true, 'cause I am a novice in this domain.

This illustrate perfectly the needs for domains where interested and awared people
can bring in their knowledge, concentrating on ONE subject at a time,
not disturbing others domains, nor beeing lost in a big overcomplicated design.

> 
> > but I think we must concentrate on the domain definition, (the global
> > view your asking for [OBSERVATIONS]) and we can at the same time
> > try to model a small well delimited concept to exchange (hopefully
> > efficiently) small amount of data (atomic value or small arrays).
> 
> Agreed.
> 
> David
> 
Pierre