revised quantity draft: comments from Aus-VO

[David Barnes]

Hi,

In the most recent data model for quantity draft (qty23.pdf, 25 May 2004),
we have spotted one problem, have one question, and one suggestion to make
arising from a review within the Aus-VO community.

1. [problem] we note the use of "sec" in sections 7.4 and 7.6.  This is 
   not an international standard unit, and should be replaced with "s" 
   assuming the authors meant seconds of time.

2. [question] the use of prefixes is not clear to us.  There are plenty
   of examples throughout the document where examples are given eg.
   "cm" and "km", but there is little information given about how units
   will be checked for validity.  Is there a clever schema which is able
   to specify the available prefixes without indicating which particular
   combinations are allowed / illegal?  A quick search for "prefix" in
   the document turned up nothing!

3. [suggestion] the question of accuracy is raised, and in 4.13 a tuple
   is given as a possibility for non-simple data types.  Looking at all
   astrometry packages, times are already always given as two doubles
   (you can barely get a us precision over a full year in one word)
   and the same is getting true for frequencies: THz central frequency
   with precise offsets.  A data model where it is possible to specify
   an offset in a separate word of a tuple would solve this (and future)
   precision issues.

We note that others may have already raised these issues/suggestions,
but unfortunately we do not have time to monitor the DM list always
to know if this is the case!

 - David Barnes (for Aus-VO).