Little data models

[DIDELON Pierre]

> From: martin hill <mchill at dial.pipex.com>
> To: dm at ivoa.net
> Subject: Little data models
> 
> I (and some other AstroGrid folks) need to start extracting and comparing data
> models in the *next fortnight* (and that's only because I'm on holiday next
> week); not in the next few months.  So I admit to being disappointed that
> neither the data modelling group nor the UCD group can help me do this!
> 
> So I need to start, but I would like to do so in the context of the VO.  So I
> would like to propose a variation on Brian's procedure.  Because while I'm sure
> he's a nice bloke, and I do know his name, and don't disagree with *everything*
> he says, I do believe his roadmap started wrong, and went wronger...  I really
> don't think we will agree any abstract models for a long long time given the 
> Quantity discussion, and I also don't think the 'level' concept is right...

The Pb with Quantity discussion was that two level/concepts were mixed, IMO.
The very simple quantity model as the one you claim, and something more
complex related with data structure. The actual pb of DM seems to me, that each 
time you believe you could model a simple item, as you start to try to model it
it become more amd more complex, because modelling domain (or area/package)
have not been defined yet.
IMHO we must start to defined common area of interest and work,
as stressed by Jonathan, and then, once the domain is clear for every body
we can share or views/experience trying to define the common understanding
in the VO-DM. During this exercice, we will certainly define basic bloc/class
which would perhaps be used in other package or domain...

> 
> First, let us assume that we all know what we are individually doing - we all
> know how our data is structured within our own disciplines/areas of expertise. 
> This is what Brian called our own namespace, and already exists.  What I (as an
> implementer) want from you experts is some specific models, so that I can use
> them when comparing data from different disciplines. In Brianspeak, we want to
> move models from individual namespaces to the VO space so I can use them
> (because as a non-astronomer, I don't have any in mine).
> 
...
> 
> I suggest that we concentrate on the oft-used specialised 'leaves' at the
> moment, as these are what we will use in practice, and see what the common
> denominators are later.  We will be able to agree much more quickly on how to
> describe a point World Co-ordinate than a co-ordinate.  Especially if we start
> with a simple one and allow ourselves to extend it later by adding components -
> for example, we might start with a simple +/- error, and worry about other error
> types later.
> 
> I also suggest that we separate out common reference data.  For example, we
> might have a WorldCoordinateFrame *instance* which includes equinox, perhaps
> epoch for groups of observations, perhaps similarly error (depending on the
> situation) - and then all coordinate instances refer to that frame instance.
> Similarly a Passband model which is referred to from brightnesses.
> 
> And finally we must either associate a UCD (or set of UCDs) with each of our
> tags or allow them to be inserted - I haven't seen anything about this yet!

See IDHA model of Mireille Louys!
> 
> My first proposed data model will be for World Coordinates as this is my
> immediate urgent requirement.  I have seen hints that such a model exists but I
> can't find it, so if it does, let me know where it is!
> 
> The second will be for brightness which is my next most urgent...
> 
> 
> -- 
> Martin Hill
> 07901 55 24 66
> www.mchill.net
> 
Pierre Didelon -----