Quantity "tables"

[Doug Tody]

> Your example is fascinating but I wonder if someone else approaching the
> same set of data would create the same hierarchical structure: ie, how easy
> is it going to be to get all astronomers to agree on the one hierarchical
> structure for the representation of astronomical data. Maybe we need to
> define all the primitives - quantity, ucd, unit, instrument, etc - and some
> of the most common and agreed simple structures like position and then come
> up with the ability for astronomers, data centre managers, developers to
> combine these into appropriate hierarchies for representing metadata. Using
> the same primitives will at least allow interpretation of results even if
> they exist in unfamiliar structures.

Tony - This is exactly what we mean when we talk about component data
models which can be aggregated and associated to model complex datasets.

We will never develop one data model for all astronomical data (one model
to rule them all?).  A realistic approach is to model obviously useful
things which are small enough that we can understand and agree upon them
(components) and then stick these together to model actual datasets.
A "spectrum" or "image" model would be assembled from these components.
A conformant dataset would supply the minimum required elements of the
dataset, but a given data provider could add other elements as well.
This would give us, for each dataset, a standard core built from standard
components, plus the ability for data providers to extend the core model
with whatever information is required to describe the peculiarities of
their data.  This would allow the structure we define in VO to evolve to
work with the real data that data providers will publish to the VO.

	- Doug