Dear Martin, All,
I was in the process of replying to this thread when Martin's latest message appeared - I'm very pleased with the conclusion that has been reached. I too am in favour of keeping the name and improving the definition, so it seems to be a unanimous decision!
I completely agree with Jonathan's point that area_cover would be straying too close to something that sounds like the meaning of the standard name area_type - indeed land_cover is an alias of area_type. We'd risk replacing one form of confusion with another if we were to go down that route.
In fact, we have quite a number of existing area_fraction names (36 in total), e.g., area_fraction_below_surface, area_fraction_of_day|night|twilight_defined_by_solar_zenith_angle, burned_area_fraction, etc., plus a whole family of cloud_area_fraction names. I think it would make sense to include Martin's new wording in the definitions of all these - I'll go ahead and do that in the next standard name table update (March 4th) unless anyone objects.
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
NCAS/Centre for Environmental Data Archival Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
-----Original Message-----
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> On Behalf Of Martin Juckes - UKRI STFC
Sent: 07 February 2019 16:58
To: Jonathan Gregory <j.m.gregory at reading.ac.uk>; cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Putting the units in a CF standard name: area_fraction
Dear Jonathan,
Thanks, that justification will be helpful in replying to people.
To summarise, the proposal (now backed by Jonathan and John -- after dropping the idea of changing the standard name) is that the current text '"Area fraction" means the fraction of horizontal area.' in the description of the standard name "area_fraction" should be replaced with the following:
"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.
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: 06 February 2019 21:23
To: cf-metadata at cgd.ucar.edu
Subject: [CF-metadata] Putting the units in a CF standard name: area_fraction
Dear Martin
I would say yes, that the use of "fraction" in area_fraction is for consistency with all the other uses of "fraction" in standard names (mass, mole, time and volume). In addition I would say that "cover" would be a confusing word to use, because "land cover" often means "land surface type". Finally, I would say to experts who are offended that in this case, as in plenty of others where CF has not quite followed familiar terminology in the domain, there is no implication that anyone thinks they are "wrong" in their terminology. It's just that CF is used across a wide range of disciplines and as far as possible all of it has to be consistent and intelligible to everyone.
Best wishes
Jonathan
----- Forwarded message from Martin Juckes - UKRI STFC <martin.juckes at stfc.ac.uk> -----
> Date: Wed, 6 Feb 2019 12:16:06 +0000
> From: Martin Juckes - UKRI STFC <martin.juckes at stfc.ac.uk>
> To: John Graybeal <jbgraybeal at mindspring.com>, Jim Biard
> <jbiard at cicsnc.org>
> Cc: "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
>
> Hello John, others,
>
>
> Thanks for those comments. I can see the value of maintaining consistency and being careful about changing things which have worked well for a long time, but I would rather not go back to the people who find the existing terminology confusing (these are people who have specifically commented on the standard name area_fraction) and tell them that we are not changing it because it has always been like that. I'd rather have a more positive message that might encourage them to appreciate the value of CF.
>
>
> I'm not sure if this is true, but it looks to me as though the formulation "area_fraction" owes something to "volume_fraction", "mass_fraction" and "mole_fraction", all of which follow wide spread usage in the atmospheric and oceanographic science communities. People who use mass and volume fractions appear to be accustomed to having these expressed as percentages outside CF, so it is no surprise to find this done in CF. For "area_fraction" we have a slightly different situation: the term doesn't arise from expressions used in the land surface science communities, rather it is a semantic structure being imposed on them. Does anyone now if this interpretation is correct (i.e. that we use "area_fraction" rather than something which might be more familiar for land surface scientists such as "area_cover" in order to maintain consistency with mass, volume and mole fractions)?
>
>
> regards,
>
> Martin
>
>
>
> ________________________________
> From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of John
> Graybeal <jbgraybeal at mindspring.com>
> Sent: 01 February 2019 07:12
> To: Jim Biard
> Cc: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Putting the units in a CF standard name:
> area_fraction
>
> Martin,
>
> I like your definition.
>
> While there is a case for renaming the standard name, it's long-time use, validity, and the fact only sophisticated data managers use standard names (and most data users just look primarily at variable names) says to me we should keep the existing standard names with fraction.
>
> John
>
> On Jan 31, 2019, at 08:07, Jim Biard <jbiard at cicsnc.org<mailto:jbiard at cicsnc.org>> wrote:
>
>
> Hi.
>
> I understand that concern, but it has always been true that the units for a quantity identified by a standard name only has to be convertible using UDUNITS from the canonical units specified in the definition for that standard name. So percent is, by definition, valid for a quantity with units of '1'. As you can see below:
>
> > udunits2
> You have: 1
> You want: percent
> 1 = 100 percent
> x/percent = 100*(x/)
>
> I guess I don't see the need for guidance here.
>
> Grace and peace,
>
> Jim
>
> On 1/31/19 10:51 AM, Martin Juckes - UKRI STFC wrote:
>
> 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><mailto:cf-metadata-bounces at cgd.ucar
> .edu> on behalf of Jonathan Gregory
> <j.m.gregory at reading.ac.uk><mailto:j.m.gregory at reading.ac.uk>
> Sent: 31 January 2019 13:20:24
> To: cf-metadata at cgd.ucar.edu<mailto: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><mailto: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><mailto:martin.juckes at stfc.ac.uk>
> To: Steven Emmerson <emmerson at ucar.edu><mailto:emmerson at ucar.edu>
> Cc: "CF-metadata
> (cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>)"
> <cf-metadata at cgd.ucar.edu><mailto: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><mailto: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<mailto: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><mailto: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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Received on Thu Feb 07 2019 - 10:14:17 GMT
