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 <[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
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