VOunits draft

[Rob Seaman]

Hi Anita,

I've removed the cc to VOTable.  I presume units fall under DM -  
although semantics could work, too.

> It was to show that we have to find a way of distinguishing between  
> M (or any other SI prefix) as an SI prefix, and M as some other  
> acronym - I could have chosen crab v. centi-rab as the example if  
> there were rabs around!  Sorry!

Rather than implementing some general purpose mechanism for  
representing SI prefixes, why not ignore the issue entirely?   
Centimeters and kilometers might be easily convertible to meters, but  
they are really three different units in practice.  That is,  
explicitly support cm, km and m as separate unit labels.

Only a very small fraction of the grid of SI units vs prefixes are  
actively used by any community in the physical sciences.  This really  
isn't an astronomical issue.  Leave SI prefixes to the elementary  
schools as a gimmick for teaching the "metric system".

> On magnitudes, there is a separate working froup looking at  
> photometry, who will hopefully come up with a standard for the  
> minimum metadata needed to identify a magnitude system and so on,  
> which Units should support.

Well, sure, but there are other logarithmic scales such as decibels.

...And there is that SI prefix issue again - do we really care to  
support centibels and kilobels too?

Rob