UTYPEs and UCDs

[David Berry]

Jonathan,

> Now Gerard and Pat (and maybe even Brian?) would say, I imagine, that
> the right way to do this in a data model is not to use UCDs, but to have
> separate classes for each physical concept. What that loses, for me, is
> the fact that for a broad group of applications the two different
> observables `flux density' and `surface brightness' and the two
> different coordinates `wavelength' and `frequency' will be handled by
> software in the same way; all we care about for many purposes is that
> it's a one-dimensional array, not what the axes are. Using a model in
> which the commonality is explicit and the differences are handled by
> having the UCD attribute with different values instead of by having
> different classes is a compromise between
> old-style-everything-hard-coded and extreme OO approach; we simplify the
> problem by having relatively few software classes and handing the hard
> work over to the UCD definition list.

I agree with this view. There are many potential classes of Frame
in which the only difference would be the associated UCD, and it makes no
sense to define different classes of Frame for them all. My only point
would be that there *are* some Frames which require extra information,
for instance a SkyFrame will have different properties to a FluxFrame.
So having a UCD as a component of Frame will cut down a lot on thenumber
of sub-classes of Frame which we need, but it will not reduce that number
to one!

> I haven't assimilated the 50 mails that happened while I was at lunch;
> more this evening.

Hope is in sight - Europe is going out to the Pub shortly!

David

----------------------------------------------------------------------
Dr David S. Berry    (dsb at ast.man.ac.uk)

STARLINK project		 |	Centre for Astrophysics
(http://www.starlink.ac.uk/)	 |	University of Central Lancashire
Rutherford Appleton Laboratory	 |	PRESTON
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