VOUnits RFC

[Arnold Rots]

Just some comments on the first part of this message...

  - Arnold

On Thu, Aug 1, 2013 at 9:59 AM, Norman Gray <norman at astro.gla.ac.uk> wrote:

>
> Greetings, all.
>
> This is a compendium reply to important points in Arnold's, Rob's and
> Rick's messages; a reply to Markus is in a separate message.
>
> On 2013 Jul 30, at 15:58, Arnold Rots wrote:
>
> > 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).
>
> That's an interesting distinction, but I think it's too complicated for a
> unit-string specification to think about -- for me, that's firmly in the
> application layer.  It's the sort of thing which might be in a discussion
> about Quantities, where there would be plenty of pointy-brackets to play
> with!
>

Hmmm, I'm not sure it's in the application layer.
It speaks to the intentions of the author: how the author intended the unit
to be interpreted;
see the comment below.


>
> > 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.
>
> It sounds as if there is indeed a case for quoting units, so that someone
> could write
>
>     kW/'martianDay'
>
> to prevent the denominator being parsed as milli-'artianDay.  However I
> don't think it's necessary to require that _any_ non-known unit be quoted,
> if only because that would require any unit writer to have memorised the
> complete set of what is and is not a known unit for the syntax intended.
>
> Remember that both of the Unity library implementations allow the
> application to ask 'is everything in this parsed unit a recognised one?',
> and to investigate (or simply object or warn) if not, and I would hope that
> this would be the same for any implementations in other languages.
>
> > 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
>
> Defining other units would be necessary, I think, but (a) couldn't
> feasibly be done _within the unit string_, and (b) would be a natural
> facility for a library implementation to provide.
>

No, not in the unit string. I'd suggest to add the option of adding an
element to
the document that allows definition of a custom unit, like
<defineUnit name="Myweight" unit="kg"> 88.1 </defineUnit>
And one could add an attribute that indicates what it's intended use is to
be
(whether the value is to be taken as absolute and literally).

Cheers,

  - Arnold

>
> ----
>
>
> All the best,
>
> Norman
>
>
> --
> Norman Gray  :  http://nxg.me.uk
> SUPA School of Physics and Astronomy, University of Glasgow, UK
>
>
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