[QUANTITY] The quandary

[Roy Williams]

> So I think we need to generalise the statement made by David that "a
> Quantity is simply a value and a unit". I think a Quantity should be "a
> value and a Domain".

Actually I like the former definition. It is clear and well-defined, it is
what we all learned in science class, it allows us to start work in a
concrete way. The point here is not expressing everything that can be
expressed, but in having a consensus among the community about what we are
dealing with.Saying a quantity is a "number + unit" is as good as we will
get!

> (I would say a UCD if I knew what a UCD actually is - I was a
> bit confused by last weeks discussion - was I alone?).

UCD provide higher level semantics. It is a "semantic type" for a data
quantitiy. For example the quantities RA and GLON are both in degrees, but
have different UCDs. The machine should say "are you sure about this?" if
you attempt to, say subtract them. However, maybe you decide it is OK to use
GLON|ERROR as an approximation to RA|ERROR.

Similarly, TEMPERATURE_CCD and TEMPERATURE_PLASMA have the same units, but
different UCD --> different semantics.

When I read about your "Domain" classes, it looks like UCDs. So now I am
getting confused!

Roy