VOUnits update: Empty/missing units

[Stéphane Erard]

Hello

> Le 3 déc. 2021 à 11:19, Mark Taylor <m.b.taylor at bristol.ac.uk> a écrit :
> 
> On Tue, 30 Nov 2021, Markus Demleitner wrote:
> 
> 
>> That's a special case, though, and when we're special-casing anyway,
>> there's something to be said for making "1" that special case.  There
>> may be profound reasons to prefer that, but a very pragmatic one
>> would be that unit="1" indicates that someone has thought about it
>> and it's not just some kind of unfilled template or other oversight,
>> and in particular that it's not something like what we have in (b).
> 
> 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.

Stéphane