VOUnits: discussion of equality of units

[Norman Gray]

Greetings.

I had some off-list discussion about the extent to which the VOUnits Recommendation does or does not -- or should or should not -- say anything about whether two unit strings should be treated as equivalent.

That is: given two unit strings "m/s" and "s^-1.m", should the Recommendation say anything about the equivalence, or otherwise, of these two strings?

My instinct would be no, and indeed that the Recommendation should be more explicit in saying that it says nothing about this.  Would the list agree? (the text below expands on this a little).

I include some clarifying text below, and propose including this in Sect. 1.4 as part of the post-RFC edits.

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There's a remark in section 2.10 about the scale factor, saying "This Recommendation does not prescribe [...] how units with scale-factors should be compared for equality" (which I think comes from an earlier on-list discussion about the syntax of scale factors).

My first reaction would be to say that this remark should be moved from Sect. 2.10 to, say, Sect. 1.4, "What this document will not do", along with a remark that the Recommendation doesn't make _any_ stipulations about equality, on the grounds that that is a higher-level, or 'application-level', consideration.  However I can imagine becoming either more or less convinced of that as I think about it more.

In any case, we should probably be explicit about this being unspecified (if that's what we conclude).

For the sake of some concrete text, how about a final bullet in Sect 1.4:

 * In considering unit strings for equality, an application SHOULD [not MUST] regard two instances of a known base unit as being equal (that is, the quantities "3m/s" and "99m/s" have equal units), but this Recommendation does not specify how an application should compare units when there are any unknown base units (for example "in/s", "'furlong'/s" or "'m'/s").  This Recommendation does not specify how to compare units with scalefactors (cf Sect 2.10).

Best wishes,

Norman


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This is part of a discussion of "Units in the VO"
IVOA Proposed Recommendation 1.0-20131224
Editor(s): Sébastien Derrière and Norman Gray

PDF: http://www.ivoa.net/documents/VOUnits/20131224/
changes: https://code.google.com/p/volute/source/browse/trunk/projects/std-vounits/VOUnits.tex
changes: https://bitbucket.org/nxg/unity/commits/all
RFC: http://wiki.ivoa.net/twiki/bin/view/IVOA/Dec2013VOUnitsRFC

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