Thank you for all the responses, it sounds like 'all of the above' is the
preferred response to my suggestions of plausible next steps. I will pursue
all of these.
Eizi's point about having no_unit in udunits is sound; I suggest we request
udunits use
'no_unit'
as a representation of
'?'
such that the behaviour is consistent; 'no_unit' should always raise an
exception when used in the udunits processing rules, exactly as '?' does.
With regard to meaning, I have found the wikipedia entry useful:
http://en.wikipedia.org/wiki/Dimensionless_quantity
`In dimensional analysis<http://en.wikipedia.org/wiki/Dimensional_analysis>, a dimensionless quantity or
quantity of dimension one is a quantity<http://en.wikipedia.org/wiki/Quantity> without an associated
physical dimension<http://en.wikipedia.org/wiki/Dimensional_analysis>. It is thus a "pure"
number, and as such always has a dimension of
1.[1]<http://en.wikipedia.org/wiki/Dimensionless_quantity#cite_note-1>'
which it has sourced from
"1.8 (1.6) quantity of dimension one dimensionless
quantity"<http://www.iso.org/sites/JCGM/VIM/JCGM_200e_FILES/MAIN_JCGM_200e/01_e.html#L_1_8>.
International vocabulary of metrology ? Basic and general concepts and associated terms (VIM).
ISO<http://en.wikipedia.org/wiki/International_Organization_for_Standardization>. 2008.
Retrieved 2011-03-22.
This is a good enough source for me.
I will wait to give space for more comments, then, if people are content, I
will raise a change request with udunits.
Assuming this is accepted and processed I will raise a change request for CF to
add some text to 3.1.
Finally I will request a change for any standard_names which appear not to
follow this approach (I have only 'area_type' so far).
I hope this seems like a reasonable response.
________________________________
From: Eizi TOYODA [toy...@gfd-dennou.org]
Sent: 31 October 2014 08:44
To: John Graybeal
Cc: Hedley, Mark; CF Metadata List
Subject: Re: [CF-metadata] string valued coordinates
Hi John
I think '?' is not a definition that is helpful to most users -- it is more
like an indication that the string -- the empty string in this case for example
-- has not provided a meaningful indication of what the units are.
I share the same impression. I was thinking it would be nicer for maintener of udunits. We should ask modifying udunits so
that it would refuse processing "no_units" otherwise ut_multiply("no_units", "no_units") returns
"no_units 2". If I remember right the unit string "?" causes immediate error, so we don't have to change
udunits.
But I'm okay if the majority here agrees that sort of thing is not a
responsibility of udunits.
Best,
Eizi
Best Regards,
--
Eiji (aka Eizi) TOYODA
http://www.google.com/profiles/toyoda.eizi
On Fri, Oct 31, 2014 at 9:45 AM, John Graybeal
<john.grayb...@marinexplore.com<mailto:john.grayb...@marinexplore.com>> wrote:
Thanks for summing this up so neatly Mark!
We could take the view that the conventions would benefit from the addition of
some text into 3.1 to explicitly make the point about quantities which are not
dimensioned or dimensionless.
We could alternatively defer to udunits as most unit questions do, which
already exhibits this behaviour, and just patch the 'area_type' and any similar
names with erroneous canonical units.
We could also request that udunits be updated with a clearer string for this
case, given the need for it, such as including the term 'no_units' as a valid
udunits term to mean there are no units here: this is not dimensionless, this
is not dimensioned.
Why is the first option exclusive to the others? Seems useful to improve the
documentation regardless.
So I agree that '1' makes no sense for area_type. I'm wondering if someone can
crisply describe what is meant when we (or UDUNITS) say a unit is
dimensionless? I'm not entirely sure I get it.
In any case, I think '?' is not a definition that is helpful to most users --
it is more like an indication that the string -- the empty string in this case
for example -- has not provided a meaningful indication of what the units are.
So my ideal solution has CF well aligned with UDUNITS, and a clear concept and
definition. Which I think suggests asking UDUNITS for a term 'no_units', defined as
"the values do not have units; values are neither dimensioned nor
dimensionless."
John
On Oct 30, 2014, at 11:06, Hedley, Mark
<mark.hed...@metoffice.gov.uk<mailto:mark.hed...@metoffice.gov.uk>> wrote:
The unit of '1' is generally used to indicate fractions and the like. In cases
where I am storing a raw binary value, I leave off the units attribute, as the
'number' isn't something that should be treated as a decimal quantity.
This is the same behaviour as I was looking to adopt, but CF 3.1 makes this
incorrect, I believe, as a lack of a units attribute is to be interpreted as a
units of '1'.
I think a clear way to define that a quantity is not dimensioned and is not
dimensionless is required. I would have liked to use the lack of a unit for
this purpose, but this has already been taken, so something else is needed.
My preference is that one explicitly puts in the units. For dimensionless, "1" or
"" is ok for udunits.
udunits2 treats '1' and '' differently.
a unit of '1' has a definition of '1'
a unit of '' has a definition of '?'
The CF conventions description of units (3.1) states that an absence of a units
attribute is deemed to be equivalent to dimensionless, a unit of '1'. This is
the convention, and it has been in force a long time.
However CF makes no statement that I can find regarding a unit of ''. Thus I
believe we defer back to udunits, which CF states is how units are defined.
Udunits states that a unit of '' is undefined, the quantity is not dimensioned
and is not dimensionless. We could adopt this to use for the cases in question.
area_type is given in the standard_name table as having a unit of 1. It is a
categorical string-valued quantity.
On the basis of the discussion, I would suggest that this is an error. If
area_type is a categorical string-valued quantity, it is not dimensionless, it
is not continuous and numerical, it is not a pure number and should not be
treated as such. I think we should fix this.
We could take the view that the conventions would benefit from the addition of
some text into 3.1 to explicitly make the point about quantities which are not
dimensioned or dimensionless.
We could alternatively defer to udunits as most unit questions do, which
already exhibits this behaviour, and just patch the 'area_type' and any similar
names with erroneous canonical units.
We could also request that udunits be updated with a clearer string for this
case, given the need for it, such as including the term 'no_units' as a valid
udunits term to mean there are no units here: this is not dimensionless, this
is not dimensioned.
I don't mind which route is preferred, I'm happy to put a change together and
pursue it; whichever way seems better to people.
cheers
mark
________________________________
From: CF-metadata
[cf-metadata-boun...@cgd.ucar.edu<mailto:cf-metadata-boun...@cgd.ucar.edu>] on behalf
of Jim Biard [jbi...@cicsnc.org<mailto:jbi...@cicsnc.org>]
Sent: 30 October 2014 16:12
To: cf-metadata@cgd.ucar.edu<mailto:cf-metadata@cgd.ucar.edu>
Subject: Re: [CF-metadata] string valued coordinates
CF says that if the units attribute is missing, then the quantity has no units.
The Conventions document, section 3.1 says:
The units attribute is required for all variables that represent dimensional quantities
(except for boundary variables defined in Section 7.1, ?Cell Boundaries?
<http://cfconventions.org/Data/cf-conventions/cf-conventions-1.6/build/cf-conventions.html#cell-boundaries>
and climatology variables defined in Section 7.4, ?Climatological Statistics?
<http://cfconventions.org/Data/cf-conventions/cf-conventions-1.6/build/cf-conventions.html#climatological-statistics>
).
and
Units are not required for dimensionless quantities. A variable with no units attribute is assumed
to be dimensionless. However, a units attribute specifying a dimensionless unit may optionally be
included. The Udunits package defines a few dimensionless units, such as percent , but is lacking
commonly used units such as ppm (parts per million). This convention does not support the addition
of new dimensionless units that are not udunits compatible. The conforming unit for quantities that
represent fractions, or parts of a whole, is "1". The conforming unit for parts per
million is "1e-6". Descriptive information about dimensionless quantities, such as
sea-ice concentration, cloud fraction, probability, etc., should be given in the long_name or
standard_name attributes (see below) rather than the units.
The unit of '1' is generally used to indicate fractions and the like. In cases
where I am storing a raw binary value, I leave off the units attribute, as the
'number' isn't something that should be treated as a decimal quantity.
Grace and peace,
Jim
On 10/30/14, 11:35 AM, John Caron wrote:
My preference is that one explicitly puts in the units. For dimensionless, "1" or
"" is ok for udunits. If the units attribute isnt there, I assume that the user forgot to
specify it, so the units are unknown.
Im not sure what CF actually says, but it would be good to clarify.
John
On Thu, Oct 30, 2014 at 2:37 AM, Hedley, Mark
<mark.hed...@metoffice.gov.uk<mailto:mark.hed...@metoffice.gov.uk>> wrote:
Hello CF
From: CF-metadata
[cf-metadata-boun...@cgd.ucar.edu<mailto:cf-metadata-boun...@cgd.ucar.edu>] on behalf
of Jonathan Gregory [j.m.greg...@reading.ac.uk<mailto:j.m.greg...@reading.ac.uk>]
Yes, there are some standard names which imply string values, as Karl says. If
the standard_name table says 1, that means the quantity is dimensionless, so
it's also fine to omit the units, as Jim says.
I would like to raise question about this statement. Omitting the units and
stating that the units are '1' are two very different things;
dimensionless != no_unit
is an important statement which should be clear to data consumers and producers.
If the standard name table defines a canonical unit for a standard_name of '1'
then I expect this quantity to be dimensionless, with a unit of '1' or some
multiple there of.
If the standard name states that the canonical unit for a standard_name is ''
then I expect that quantity to have no unit stated.
Any deviation from this behaviour is a break with the conventions. I have code
which explicitly checks this for data sets.
Are people aware of examples of the pattern of use described by Jonathan, such
as a categorical quantities identified by a standard_name with a canonical unit
of '1'?
thank you
mark
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