VOUnits update: Empty/missing units

[Mark Taylor]

On Fri, 3 Dec 2021, Stéphane Erard wrote:

> > I take the point about distinguishing dimensionless quantities
> > from "unit not applicable" and "author hasn't thought about units".
> > I don't think that using the empty string as distinct from null is
> > suitable for this (or anything else), since those two inevitably
> > get confused with each other (as above).  Given that, allowing "1" in
> > the syntax for that purpose is probably reasonable for metadata authors
> > who want to make the point that a quantity really is dimensionless.
> > But I don't think it's reasonable to expect every dimensionless
> > value to supply an explicit indication of that; mostly it won't
> > serve any useful purpose and authors probably won't do it anyway.
> > It can also get in the way; one obvious thing that people do with
> > units is quote them for human consumption, e.g. label an axis.
> > In that context "count" is more readable than "count / 1 ».
> 
> I agree with Mark - using ‘1' will be an issue for plotting. 
> But ‘count’ is not better. E. g., there are many relative spectral scales in reflected light which are dimensionless but are not counts. Besides, some dimensionless quantities use units (angles…).
> Using ’No unit’ may be a best choice.

Misunderstanding?  I might have phrased my example badly.
I wasn't suggesting to use the value "count" as a unit representation
to mean dimensionless.

I was talking about an example case where there is a dimensionless
column having the name "count".  A plotting tool like topcat writes
the label "<column-name> / <units-text>" (if <units-text> is provided)
along the axis, so if the unit text is set to "1" the user will see
"count / 1".  If the unit text is left blank the user will just see
"count", which I'd say is a better outcome.

--
Mark Taylor  Astronomical Programmer  Physics, Bristol University, UK
m.b.taylor at bristol.ac.uk          http://www.star.bristol.ac.uk/~mbt/