VOunits draft

[Rob Seaman]

Hi Francois,

> Looks like there is a misunderstanding there -- I was just saying
> that we have to accept the full SI, with all its prefixes.

I've have been attempting to make three points:

1) Why do we "have to"?  What are the requirements pertinent to VO use  
cases that would lead to our accepting "yotta" and "yocto" (for  
instance) for our purposes?

> If an enumerated list is easier to handle in your code, please do so,
> but again that's not how the SI is defined.

2) It was implied by others in this thread that VO applications would  
need a rules-based parser.  I wanted to suggest that an enumerated  
list might be better adapted to the purpose.

> ... I really don't see the point. It may happen that a problem
> happens in floating point values when the exponent is too large
> (e.g. it's impossible to store in an IEEE floting-point number
> the value representing the mass of a galaxy express in grams);
> is that this problem you are tackling ?

3) It was suggested by others in this thread that SI prefixes somehow  
supplied a solution to numerical precision issues.  This is an  
interesting idea, but devoid of details.  How would this work in  
practice?

> Anyway the discussion on units is taking place to-morrow;
> and I really hope that the ISO and IAU recommendations will
> be viewed as more compelling than what can be read in a
> novel -- even if the author is an excellent scientist.

The IVOA ultimately falls under commission 5 of the IAU.  We should  
certainly be cognizant of - and influential over - the development of  
IAU standards.

This is the first time the general preeminence of ISO standards has  
been suggested in this thread.  Perhaps you meant SI (CGPM).  We do  
have some interest in a subset of ISO-8601, but the proprietary nature  
of ISO isn't a good match to either IAU or IVOA.

IVOA standards requirements should derive from use cases, not from an  
appeal to any external authority, whether IAU, ISO or SI.

For example, SI base units are the metre, kilogram, second, ampere,  
kelvin, mole, candela.  Astronomers use all of these at some point,  
but use a wider variety of non-SI units for key purposes.  It is  
simply not true that SI is all we need to worry about.  Since we're  
only interested in a subset of ISO-8601, perhaps we're also only  
interested in a subset of SI, perhaps deprecating the more obscure SI  
prefixes.

Rob