Ed, (01)
This discussion raises some serious questions. (02)
JFS>> And finally, if we develop a UoM ontology that differs from that
>> document in any way, then our UoM is the one that is *wrong* --
>> because all the users take that document as the Bible, not ours. (03)
EB> I take issue with this. The VIM is a standard vocabulary. It
> does not represent itself as a formal ontology... (04)
Yes, I agree that a formal ontology would have a different structure.
I should have emphasized that the required agreement is at the level
where it is directly relevant to applications that use those units.
(But different applications will have different requirements.) (05)
EB> In particular, the VIM explicitly says that the magnitude of a
> quantity is not knowable; it is only measurable, and every measurement
> involves uncertainty. This is absolutely true, and it is the gospel
> of the science. (06)
Not just the science of measurement, but *every* empirical science.
This gets into serious questions of how granularity and uncertainty
should be accommodated in an ontology. (07)
EB> But it means that you cannot say that a stretch of roadway is 1km
> in length; you must say that it is 1km plus-or-minus 5 cm, if that
> is the accuracy of your survey. And similarly, the label on your
> Coke bottle that says "1 litre" and the label on your bag of
> potatoes that says "10 lbs" is, according to the VIM, meaningless
> without additional information. (08)
Such details are extremely dependent on the application -- they
belong in the microtheories. For example, if somebody's height
is listed as 180 cm, the granularity would not be relevant in a
medical record. But if an airplane part has a length of 180 cm,
a subcontractor that manufactures it must know the tolerance. (09)
EB> And yet, we compare quantity values all the time, without
> apparent concern for the uncertainties of the associated
> measurement process. (010)
The word 'apparent' is significant. The reason why we don't seem
to be concerned is that we are always using "common sense", which
is extremely context dependent. The nurse who enters 180 cm on
a medical record knows that the measurement will change with
posture or haircut. The airplane designer, however, knows that
a given slot has a narrow tolerance for a rigid slot-filler. (011)
EB> Are we making a formal ontology for measurement science,
> or a practical ontology for units of measure? (012)
Different kinds of "practical problems" will require different ways
of treating (or ignoring) the tolerance. It's probably impossible
to accommodate all the ways in a single, fixed formal theory. (013)
This is one more example why I have said that an upper level ontology
must have very few axioms -- in fact, it should be much closer to a
terminology than a formally axiomatized theory. (014)
The details belong in microtheories. We can have several microtheories
for different ways of dealing with granularity or tolerance. (015)
John (016)
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