[QUANTITY] The difference between quantities and measurements (Was: Re: [QUANTITY] Why quantities always have errors (Was: Re: [QUANTIT] Use-cases, role in larger scheme)

[Brian Thomas]

	Martin,

On Monday 17 November 2003 06:08 pm, Martin Hill wrote:
> >       This might be true if you never compare measurements with
> > calculations, or simulations with calculations or so on. But that isnt
> > the case, is it? We have a very real need to compare measurements with
> > calculations or with "declared values" (such as a theorist defining the
> > speed of light to be "1").
> >
> >       Give me a use-case or two where you need to limit the use of the
> > quantity by it origin AND how that would change the calculation (or a
> > cut, paste op).
>
> I think we're back to the sources of values not the values (and perhaps
> the cross-purposes of our discussion).  You can simulate a star, and you
> will get a flux (I presume without an error unless you're simulating a
> sample of stars or you're building errors in specifically?).  Or you can
> observe a star, and get a flux.  Both fluxes are the same 'type' and you
> can compare fluxes.  You can't compare the position of the star with the
> simulated flux, and there should be *no way* of representing such a
> comparison in our data models without some big flag coming up and
> hitting the developer over the head.  Nor can you compare measured flux
> and measured position.  You should not be able to cut and paste a flux
> into a position plot.  You will want to be able to serialise both - but
> you'll want to serialise a lot of things that aren't quantities.

	I agree with all of the above. 

>
> And what I'm trying to say is: I don't think there is anything common
> between the type 'flux' and the type 'position' that merits having a
> 'quantity'.

	But there are commonalites. Think for a minute on the package 	
	that must handle passing around these things. Or searching for
	the values of these things, or breaking apart one of these things
	to make many children (big position quantity broken into smaller
	position quantities) and you will begin to see that the same operations,
	treatment are needed over and over again.


	Regards,

		=b.t.

-- 

  * Dr. Brian Thomas 

  * Code 630.1 
  * Goddard Space Flight Center NASA

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