Re: [unitsml] Identity of units

["Martin S. Weber" <martin.weber@nist.gov>]

Quoting "Peter J. Linstrom" <peter.linstrom@nist.gov>:

> All,
>
>   I've added Joe Solsky to the distribution list as
> noted in the agenda of Wednesday's teleconference.

Ooops, sorry Joe to forget about you.

>
>   I just have a few comments about the proposed approach.
>
> 1.) The SI base, special derived units, and non-SI units
>    approved for use with the SI are all enumerated in
>    UnitsML, so units which are derived from them should
>    generally not need to use the <ExternalRootUnits>
>    element. (...) In any
>    case I believe such usage [of External instead of Enumerated
>    Root Unit] should be discouraged

I fully agree

>    and that
>    there should be a need to resolve <ExternalRootUnit> URNs
>    to deal with units which in the SI or approved for use
>    in the SI. I believe this would also be largely true for
>    units in the cgs and U.S. and U.K. customary unit systems.

I'm not sure I agree with this one. Why would we want to limit  
unitsml's scope to that of SI units and their friends only? Even if  
I'm marking up a unit whose use is discouraged by the SI, shouldn't I  
use unitsml for it?

>
> 2.) The guidelines should mandate that enumerated root
>    units be used whenever possible. (...)

Absolutely!

> 3.) The purpose of the <ExternalRootUnit> element is for
>    units which can't be defined in terms of other units.(...)

Is it? I got the impression it's for units that indeed can be defined  
in terms of other units, but whose underlying units are not in the  
list provided by <EnumeratedRootUnit> (either because it's non-SI or  
because it's itself a synthesized unit based on <EnumeratedRootUnit>s).

Also the above all helps with determining the identity of derived  
units, what is a hard issue for me atm is determining the identity of  
two unit definitions of the same base unit (which will most likely  
happen because not everybody's using UnitsDB / people do not need to  
use UnitsDB). Any thoughts on that?

-Martin