VOUnits RFC

[Arnold Rots]

There is something else that bugs me about this issue and that
became apparent to me in a side conversation with Roy Williams:

There are two ways custom units can be (intended to be) used:

As a handy, well-known unit (handier than standard SI units) - like
solar mass, earth mass, speed of light, etc.
Or as a handy scalar whose exact value is not (yet) known - like
the Hubble constant.

So, in addition to the question what the scale factor is (or, more
precisely, how the custom unit is to be converted to SI units),
there is the question whether the author intended to use the custom
unit as an SI surrogate or whether (s)he merely wants to provide
a ratio (like: this planet's mass is 5.23 Jupiters and I don't care
what that is in kg).

This further muddies the waters and leads me to prefer that only
units from Norman's tables be allowed.
If that is not feasible, then creative units should be quoted and not
linked to SI units.
If that's not good enough, then we need an explicit mechanism to define
a custom unit in terms of allowed units, like 'H' = 75 km/s/Mpc

In rummaging around in the table I noticed that solar units are
included, but no corresponding geophysical ones, like earth mass
and radius.

Cheers,

  - Arnold

-------------------------------------------------------------------------------------------------------------
Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
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On Tue, Jul 30, 2013 at 10:10 AM, Norman Gray <norman at astro.gla.ac.uk>wrote:

>
> Greetings, all.
>
> On 2013 Jul 29, at 22:11, Arnold Rots wrote:
>
> > If Norman introduces MyWeight, how does the user know what value
> > Norman actually used?
> > Some canonical value? his preferred weight? his weight of the day?
> > Is that value (whatever it is) published anywhere?
> > The tropical year is another example.
>
> ...as is local solar day.
>
> I think this is a great example.  If I've sent Arnold a dataset with units
> of MyWeight, then it's presumably because he asked me to, or because I have
> some other grounds for believing that he'll want that, and be able to do
> something with it.  Neither of us would welcome the VOUnits spec's
> interference in that conversation.
>
> This example, and Rob's pointing out that something like jupMass may be
> recalibrated in future, are both examples of the scalefactor changing in
> time.  As such, they show that a unit is not straightforwardly reducible to
> a scalefactor*base-unit, and indeed that it might be _incorrect_ to force
> it to be so reduced.
>
> When I was implementing the VOUnit stuff to check the grammars and design,
> I realised, somewhat to my surprise, that all that a list of 'known units'
> adds is disambiguation plus a bit of extra metadata.  Syntactically,
> 'known' or 'not-known' makes very little difference.  That means that the
> question of what to do with unknown units gets punted to the application
> which is calling the parser, which is probably in much the best place to
> decide on the best course of action.  That action may consist of "I don't
> recognise this unit, so you FAIL", but it doesn't have to be.
>
> ----
>
> All of this does of course leave the question of how one communicates the
> meaning of these units.  Marco sketched a mechanism based on the VOTable
> LINK element: that's a nice approach which would work in that context, and
> which would tie in neatly to the mention of QUDT in an earlier message of
> this thread.  But it also leaves the problem of 'odd' units where it
> belongs, at an application level rather than in an IVOA Recommendation.
>
> ----
>
> Incidentally (and finally), I should point out that the unit 'MyWeight'
> would be parsed, according to the VOUnits spec, as the mega-'yWeight',
> which is pretty clearly undesirable (the consequence of this hadn't fully
> struck me before).   There are two alternatives: (i) allow units to be
> quoted (thus <'jupMass'/hr> or <'MyWeight'/'USD(Au)'>), or (ii) forbid
> prefixes on all unknown units.  Option (i) has the advantage of
> highlighting that a unit is 'unknown', but right now, I'm inclined to
> invert the prescription of the spec and go for (ii).
>
>
> --
> Norman Gray  :  http://nxg.me.uk
> SUPA School of Physics and Astronomy, University of Glasgow, UK
>
>
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