VOunits draft

[Hervé Wozniak]

Hi Mireille (et al.),

There is a (minor) point I misunderstand in the document about Units.

It is stated in Section 2.1 that the need for a Units model is partly 
driven by the need to distinguish between the use of, for instance, 
meter for wavelength and meter for distances. However, if I properly 
understand Figure 2, the UCD string that helps to make this distinction 
is attached to the Quantity class, not the Unit class. For me, there is 
a contradiction between these two parts of the document.

 From my point of view, the 'meaning' of a measurement is different if 
the measurement is a spatial distance or is a wavelength. This is indeed 
partly related to the way to get the measurement (but not always). So 
the attachment of the UCD to the Quantity model is correct.

But the meaning of the unit does not change : a spatial distance 
measured by a ruler or a wavelengh measured by reference to a 
calibration lamp is physically a length... so uses the same unit. Many 
other examples obviously exist (Hz, cycle/second, etc.)

So, if I'm right, I'd suggest to simply remove the point 3. of Section 
2.1 since this is a use case for the Quantity model and not for the 
Units model.

Other comments maybe during the interop...

Cheers,
Hervé


Le 19/05/2009 23:07, louys at alinda.u-strasbg.fr écrivait :

> Hi all,
> 
> Here is the VO UNITS draft, that I circulate for comments within the DM 
> and VOTable Working group .
> Most of the document was prepared with Anita ,Sebastien, and I.
> I put a list of authors as a first guess, but of course , you are very 
> welcome to contribute and  help this effort to converge.
> 
> I apologize for the delay.
> Hope to have your comments very soon as the discussion on the topic on 
> the DAL list was rather lively.
> Cheers, Mireille
> 

-- 
Herve WOZNIAK

Astronome
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