Dear Jonathan,
we could certainly take that approach, though the definitions are not always accessible to people looking at the standard name, so they do not compensate for ambiguity in the name itself. The current text '"Area fraction" means the fraction of horizontal area.' could be replaced with "Area Fraction" is a dimensionless number representing a relative or proportional area. It may be expressed as a fraction, percentage or any other unit that conforms to "1". It is evaluated as the area of interest divided by the grid cell area, scaled for the units chosen. I still feel that there is a case for changing the name to, for example, "relative_area" in order to reduce confusion caused by people who assume that a fraction is a quantity that does not have units, regards, Martin ________________________________ From: CF-metadata <[email protected]> on behalf of Jonathan Gregory <[email protected]> Sent: 31 January 2019 13:20:24 To: [email protected] Subject: [CF-metadata] Putting the units in a CF standard name: area_fraction Dear Martin I'd rather we retained "fraction" in the standard name, because it's always been there, it's used in other contexts in a consistent way, and there isn't anything actually incorrect with it, as you say. Could we instead add a note to the definitions pointing out that percent is acceptable as a unit for them? Best wishes Jonathan ----- Forwarded message from Martin Juckes - UKRI STFC <[email protected]> ----- > Date: Wed, 30 Jan 2019 22:40:12 +0000 > From: Martin Juckes - UKRI STFC <[email protected]> > To: Steven Emmerson <[email protected]> > Cc: "CF-metadata ([email protected])" <[email protected]> > Subject: Re: [CF-metadata] Putting the units in a CF standard name: > area_fraction > > Hi Steve, > > > The issue is more that CF allows more freedom in the choice of units than > many people expect from a "fraction". > > > A second problem, I think the problem is that I didn't explain the issue > clearly. In the CMIP data request we are specifying that variables with > standard name "area_fraction" should be given as percentages. This is allowed > by the CF convention: an "area_fraction" can be 0.5 or 50%. The reason that > percentages are being used is because "area_fraction" is being used like the > proportion of land covered in grass, and people are used to having these as > percentages rather than fractions. It is all perfectly correct as far as the > convention goes, but people often interpret the use of "area_fraction" for a > percentage as an error. > > > Given that we have the framework of allowing flexibility in the choice of > units, I feel it would be better to avoid having the term "fraction" in the > standard name, given that it is often interpreted as implying a specific > choice for the units. > > > regards, > > Martin > > > ________________________________ > From: Steven Emmerson <[email protected]> > Sent: 30 January 2019 21:37 > To: Juckes, Martin (STFC,RAL,RALSP) > Cc: CF-metadata ([email protected]) > Subject: Re: [CF-metadata] Putting the units in a CF standard name: > area_fraction > > On Wed, Jan 30, 2019 at 12:54 PM Martin Juckes - UKRI STFC > <[email protected]<mailto:[email protected]>> wrote: > > I'm afraid I don't understand your comment. When I search for "fraction" in > the NIST document I find it defined as being a ratio, which is inconsistent > with the current CF usage. The CF standard name concept "area_fraction" is > not what NIST or others understand as a "fraction". I'm suggesting a change > to remove this inconsistency. > > Unless we're talking past one another, I'll have to disagree. The NIST unit > for "mass fraction" is "1" -- even though it's a ratio. A fraction can be > represented many ways. "1:2", "1/2", and "0.5" all represent the same > fraction, for example. > > Does the CF convention require a particular representation for a fraction? > > Regards, > Steve Emmerson > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata ----- End forwarded message ----- _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
