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Subject: UnitSystem/@type : possible values


As we've talked about pre-populating the UnitSystem/@type attribute with 
predefined values (to be union'd with either an xsd:string or a xsd:token), 
here are some suggestions for possible values (way more and more fine-grained 
than likely helpful, see end of message for a boiled down suggestion):

- SI (probably too general)

- non-SI (probably too general)

- SI base (or SI_base or SI-base) (cf. NIST SP330-2008; 1.2; p.10)

- SI derived (or SI_derived or SI-derived) (cf. NIST SP330-2008; 1.4; p.12 & 
2.2, p.23)

- SI coherent derived or SI coherent (cf. NIST SP330-2008; 1.4; p.12 & 2.2; p.23)

- SI special or SI special derived or SI special coherent or SI special 
coherent derived (cf. NIST SP330-2008; 2.2.2; pp. 24,25)

- non-SI acceptable or accepted (or ..) (cf. NIST SP330-2008; 4.1 / Table 6; 
pp. 31,32)

- non-SI experimental(ly determined) (cf. NIST SP330-2008; 4.1 / Table 7; pp. 
33; 34) [!! "Because the quantity systems on which these units are based 
differ so fundamentally from that on which the SI is based, they are not 
generally used with the SI, and the CIPM has not formally accepted them for 
use with the International System" -- p.33 !!]

- non-SI temporarily acceptable (cf. NIST SP330-2008; 4.1 / Table 8 & 9; pp. 
32; 35; 37)

- non-SI non acceptable (cf. NIST SP330-2008; 4.2 ; p. 37 & 
http://www.bipm.org/en/si/si_brochure/chapter4/conversion_factors.html [btw, 
this ends up on physics.nist.gov -- isn't the world a small place?])

- (SI) acceptable units (cf. NIST SP811-2008; 5.4; p.11 : "The term 
'acceptable units', which is introduced in this Guide for convenience, is used 
to mean the SI units plus (a) those non-SI units accepted for use with the SI 
(...) and (b) appropriate multiples and submultiples of such accepted non-SI 
units")

  -----------

about the term "coherent": (all following quotes from NIST SP330-2008; 1.4 p. 
12, typos mine; not Taylor/Thompson's)

"Derived units are defined as products of powers of the base units. When the 
product of powers includes no numerical factor other than one, the derived 
units are called coherent derived units"

Note: numerical factor, not exponent. Clarifying this is the note on the word 
coherent further down in that paragraph: "The word coherent is used here in 
the following sense: when coherent units are used, equations between the 
numerical values of quantities take exactly the same form as the equations 
between the quantities themselves. Thus if only units from a coherent set are 
used, conversion factors between units are never required."

"The base and coherent derived units of the SI form a coherent set, designated 
the set of coherent SI units"

  -----------

what I don't really understand is where to fit in multiples and submultiples, 
as they don't seem to be (explicitely) in either of the mentioned ones so far 
(except for the broad "SI" one).

  -----------

So for my personal preference (disclaimer: I'm no SI expert!) I think for 
UnitsML fitting choices would be:
SI, non-SI, SI base, SI derived, non-SI acceptable, non-SI temporarily 
acceptable, non-SI non acceptable
(not necessarily in this wording)
plus "US", "inch pound" or whatever the US calls "their" system.

SI and non-SI cover basically all the units (if you want or have to remain 
vague); SI is partitioned into SI base and derived (I'm leaving out coherent 
derived as even in the committee the meaning of 'coherent' wasn't inherently 
clear); non-SI is partitioned into the accepted, temporarily accepted and non 
accepted classes.

If that's what the (outside of the TC's scope lying) UnitsDB is using is 
another question -- but for UnitsML I think these 7 (8) values would be a good 
population for the enumerated list in UnitSystem/@type.

Regards,

-Martin


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