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 <cf-metadata-bounces at cgd.ucar.edu> on behalf of Jonathan Gregory <j.m.gregory at reading.ac.uk>
Sent: 31 January 2019 13:20:24
To: cf-metadata at cgd.ucar.edu
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 <martin.juckes at stfc.ac.uk> -----
> Date: Wed, 30 Jan 2019 22:40:12 +0000
> From: Martin Juckes - UKRI STFC <martin.juckes at stfc.ac.uk>
> To: Steven Emmerson <emmerson at ucar.edu>
> Cc: "CF-metadata (cf-metadata at cgd.ucar.edu)" <cf-metadata at cgd.ucar.edu>
> 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 <emmerson at ucar.edu>
> Sent: 30 January 2019 21:37
> To: Juckes, Martin (STFC,RAL,RALSP)
> Cc: CF-metadata (cf-metadata at cgd.ucar.edu)
> 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 <martin.juckes at stfc.ac.uk<mailto:martin.juckes at stfc.ac.uk>> 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
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----- End forwarded message -----
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