[QUANTITY] Units

[Clive Page]

On Wed, 21 May 2003, Jonathan McDowell wrote:

> While in Cambridge I wrote up some simplistic thoughts on
> how to model units, which I'm going to throw in as yet another
> take on one small aspect on the quantity thing.
> I have a short document at
>  http://hea-www.harvard.edu/~jcm/vo/units/units.ps

Thanks Jonathan, for the shortest document so far on the subject.

Now that most participants are either asleep or overwhelmed by the email
flurry, I'd like to suggest that we restrict the scope of this discussion.

There's a fairly urgent need to do simple units conversions for
user-interfaces and things like SIAP and cone-search services: typically
the user will want to enter angles (to take an important example) in
hours:mins:secs or degs:arcmin:arcsec while the datasets will have their
angular units declared as "radian" or "degree".  It has occurred to many
people over the years that it would be nice, and quite easy, do do such
conversion automatically and apply factors (pi/180) or whatever, instead
of hand-crafting each and every data service.  I think that this is a
sufficiently important special case that we ought to be able to devise a
notation to make this work, so we can implement it around the whole VO.

Of course everyone then thinks: wouldn't it be nice to extend this to
convert keV or Angstrom units to Hertz, or parsecs to metres.  Which then
opens a whole can of worms (e.g. do we keep derived units like ergs and
Jansky or expand them all into fundamental MKS units every time, do we
allow any or all prefixes or just powers of three, and so on).  These
discussions could run and run.

So I'd suggest we leave the general solution of this problem to the longer
term, and just try to find a simple notation which will cope with the very
simplest cases.


-- 
Clive Page,
Dept of Physics & Astronomy,
University of Leicester,    Tel +44 116 252 3551
Leicester, LE1 7RH,  U.K.   Fax +44 116 252 3311