Follow up for my question in Characterisation Data Model RFC

[Fabien Chereau]

Hello,

This was my question on the twiki 
(http://www.ivoa.net/twiki/bin/view/IVOA/CharacterisationDataModelRFC):
> "Hello, instead of having many collections of Properties for each
> instance of Characterisation (like Resolution, SamplingPrecision
> etc..) one for each axis as shown in Figure 4, why don't you have
> only one collection of a CharacterisationAxis class which itself
> would contains the properties for this axis? I think it would make
> the design clearer. It is also convenient because to each
> CharacterisationAxis instance can be associated a single coordinate
> frame which also apply to each property."
> 
> * Response (by IVOA.Mireille.Louys): The XML representation of the
> model has this feature , that is each CharacterisationAxis is a node
> underwhich we have the properties: coverage, resolution, Sampling for
> this axis. 

> For the model we want to keep the symetry , the
> matrix-like design that is shown in the different examples. One could
> search / group the metadata in a Property first order and have , for
> example Resolution , for all axes, then Coverage, for all axes,
> etc... 

I don't understand why we would want to order the metadata in a 
Property-first order since knowing what the value of the meta data is 
without knowing for which CharacterisationAxis it is associated is not 
very useful. For example in figure 4, if I go through the list of 
Resolution and find that the first element has a resolution of 5.78, it 
is meaningless if I don't know in which CharacterisationAxis (and thus 
coordinate system and unit) it is. So at the end we always need to link 
both, and it looks simpler to me if we decide that the properties for a 
CharacterisationAxis are just its attributes.

> When the axes are not independant, that is Space depends on
> time, for instance, Resolution may be represented by a multi-variable
> function of all axes, and will be hooked in the Resolution Class.

In the general (and real) case where the axes are not independent, 
having only one network of inter-connected generic CharacterisationAxis 
classes (with methods or attribute such as getCentralPosition, getBounds 
etc.) will be easier to manage than having many Property classes 
connected to each other. Especially because the properties of one axis 
are all derived from the same quantity (for example the bounds are just 
the min/max of the support, the Central Position is just the barycenter 
of the support etc..).

Sorry if what I say is not very clear, I will try to explain what I mean 
with more details in a future email.

Fabien