[uf-new] Measurement brainstorming

[Guillaume Lebleu]

There has been already a lot of work in researching measurement 
examples, existing practices and brainstorming. 
http://microformats.org/wiki/measure-brainstorming

One of the main issue that I remember we could not agree on was how to 
deal with composite measure units.

The simplest approach is to mark up composite measure units as if they 
were elementary units. For instance a gram is really a composite unit 
(1/1000 * 1 kg), but we would nonetheless mark up "500 g" as "500<abbr 
class="unit" title="gram">g</abbr>" for simplicity purpose. The problem 
is the large amount of units that would result from this (see UNECE list 
http://www.unece.org/etrades/units.htm) and the difficulty of a program 
to do conversion.

The other approach is the one used by XBRL 
(http://microformats.org/wiki/currency-formats#XBRL) where composite 
units are explicitly marked up as composite. This has the advantage of 
making comparison and conversions much easier (Of course, in the example 
above (500 g), this approach would not work since "1000" does not appear 
in the human-readable part of the content). But forcing every single 
composite unit to be marked up explicitly would be too complex for broad 
adoption.

One way I see that may allow to resolve this conflict would be to follow 
the XBRL approach and have both a microformat for:

    * defining elementary/atomic units. ex. "kg is the SI unit of mass".
    * defining composite units as composition of other units and scaling
      factors, for instance to mark up "One gram (g) is 1/1000th of a kg".
    * referring to these definitions when marking up content such as
      "500 g".

The most used elementary and composite definitions could be anywhere 
(ideally on the Web pages of the organizations who define them), but I 
think it may be useful to put the most used ones on the microformat wiki.

Guillaume