[QUANTIT] Use-cases, role in larger scheme (Was: Re: [QUANTITY] Quantity "arguments")

[David Berry]

Doug,

> Probably it will be difficult to make any real progress on 'quantity'
> until the architecture it fits into has been clarified so that we know
> how it will be used.  To continue with this now the best strategy might
> be a simple prototype of some sort that is usable as a component (i.e.,
> the bottom-up approach).

By "bottom up" do you mean looking at features of a Quantity which we know
will be needed, even without looking at any use-cases? Here I'm thinking
of things like the need for a Quantity to contain some form of definition
of what its data values represent (i.e. does the Quantity contain a
list of flux values, positions, ccd counts, or what). It's hard to see how
a Quantity could be used if you cannot tell what it contains. So we do not
need to spend a long time working through a set of use cases to decide
that this is a requirement.

Also, it seems obvious to me that a Quantity will need to carry around
information about the location of the data values in any relevant
coordinate systems (i.e. "WCS" in the broadest sense). Without this how
are we going to compare Quantities (for instance, to re-project a set of
images onto a common projection)?

Do we really need to spend a long time working through a list of use case
to decide that these are requirements? I'm with Doug when he says that we
should work from the bottom up as well as from the top down (albeit Doug
may disagree with my previous two paragraphs - particularly about WCS).

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
DIDCOT				 |	United Kingdom
United Kingdom			 |	PR1 2HE
OX11 0QX