[QUANTITY] Do we need it? Some quantities

[Brian Thomas]

	Hi Martin!

On Monday 27 October 2003 05:40 pm, martin hill wrote:
> Being a bit radical here, but what is quantity for?  I've read the list
> (honest) but it strikes me that really a quantity is simply a number.  A
> range for example presumably consists of two quantities.  So perhaps we
> should start further back and look at what kinds of quantities we want to
> describe - personally I suspect we'll end up with a bunch of different
> 'quantity models' some of which are not necessarily related...

	Quantity is needed in order to faciltate the search, exchange, and 
	data fusion in the VO (my opinion). With that premise then, you need
	some sort of underlying framework to all the "top level" data models in order
to 
	besure that one piece of information (typically a number) is on the same 
	footing (or not) as another in a different data model.

	Overall, yes, it should be relatively simple affair. The devil is in the
detail
	that is desired, ie which things should quantities be able to describe.
	Just simple (single) scalar numbers? What about vectors? What about
	tables? Images? N-dimensional data? How are the units to be associated? 
	What about types of accuracy?
	
	These are all important questions for quantity (and, of course, there are
more..)

> Including quality is a scary topic (for me as an implementer!). Can we make it a
> separate [Quality] model altogether?  It will get attached to all kinds of
> things, including presumably complete data sets?

	Im not sure what you mean by "attached", but the quantity *is* supposed to 
	be used by the "top" level data models, and their common components via
	an inheritence relationship in the Object-oriented sense. Not all parts of
your 
	data model need inherit from quantity. BUT if you want to insure
	interoperabilty of some part, then it should. 

	Disclaimer: All points are of course my view, but they do appear to be shared 
	by a large number of other modelers I met at IVOA. Capturing requirements 
	will help clear up whether this is the case or not.

	Hope that answers your questions.


	Regards,

	-b.t.