[CF-metadata] physical vs dimensional units

From: Jonathan Gregory <j.m.gregory>
Date: Sat, 2 Apr 2011 12:42:52 +0100

Dear John

> udunits is a "dimensional units" library, for manipulating powers of
> the fundamental dimensions (length, mass, time, charge,
> temperature). this is necessary but not complete for capturing the
> meaning of the "physical units" of our data.
>
> We also need units like kg/kg, for which the udunit canonical string
> is the empty string. But even more difficult is "atoms CO2 / atoms
> air" or "grams CO2 / grams air", or "count of phytoplankton".

I don't really agree with this. Units are units, not descriptins of quantities.
grams of CO2 per grams of air is a mass mixing ratio and is dimensionless.
area fraction of sea ice is a ratio of areas and is dimensionless. These
quantities have the same (non-)units but we shouldn't expect the units to
distinguish them anyway. That is the purpose of the standard name in CF.
Similarly, many quantities have units of W m-2 or kg m-2 s-1 and are
distinguished by their standard names.

> also, all of our space/time units arent dimensional units, they are
> all referenced to a datum. we include the datum in the udunit string
> for time, but not for vertical or horizontal coordinates. thats not
> a particular problem, but it does point out that these units are not
> the same as dimensional units. we need to include the datum in the
> "physical units" representation, which could be one string or
> several strings .

Again, I think the distinction between a quantity which has a datum and one
which doesn't is really part of the definition of the quantity and can be
implied/stated in the standard name. I would say that the standard name of
time should only be used for something with a time datum. If it's an interval
of time, without a datum, it should have a different standard name. Such a
standard name has not yet been requested, I don't think. It could be
elapsed_time, for example, in general, but I note that forecast_period is such
a quantity. It has time units which don't include a datum with "since".

Best wishes

Jonathan
Received on Sat Apr 02 2011 - 05:42:52 BST