Dear Mark, Jim, Martin
Following my posting from last Fri
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2014/057810.html
I agree with Jim that to define no units is overkill for this problem.
There are quantities for which units are inapplicable i.e. string-valued ones
and flags encoding those. Jim, I wonder what "non-numerical binary quantities"
you put in CF datasets?
Is it not an adequate solution to change the document to say that units are
not defined (neither by default nor explicitly) for those classes of quantity?
Then any units att which they have is non-CF and can be ignored. This is
backwards-compatible because it doesn't make any existing dataset illegal, and
it's what you would do with the units anyway (i.e. ignore them). Does that
make sense?
Martin suggests that in CF-2 we might mandate the units att, or distinguish
between units and no units. This would be backwards-incompatible. We could
make backwards-incompatible changes in CF-2 (according to the rules we have
agreed for that discussion) but I'm not convinced that this issue justifies
backwards-incompatibility. What serious problem does it cause?
Martin also remarks: I guess it would help if udunits would accept "terms that
evaluate as '1' with the understanding that they are equivalent to "1" as
unit. I think that is the case, isn't it? Using udunits-1 at the command line:
$ udunits
You have: kg/kg
You want: 1
<1> = <kg/kg>*1
<1> = <kg/kg>/1
Has this behaviour changed in udunits-2?
Best wishes
Jonathan
----- Forwarded message from "Hedley, Mark" <mark.hedley at metoffice.gov.uk> -----
> Date: Tue, 4 Nov 2014 16:38:12 +0000
> From: "Hedley, Mark" <mark.hedley at metoffice.gov.uk>
> To: Jim Biard <jbiard at cicsnc.org>, "cf-metadata at cgd.ucar.edu"
> <cf-metadata at cgd.ucar.edu>
> Subject: Re: [CF-metadata] string valued coordinates
>
> Hello Jim
>
> > A variable with no units attribute at all is also pretty unambiguously a marker for something that isn't intended to be a even a pure number.
>
> If only this were the case. CF conventions state that:
> 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.
> http://cfconventions.org/Data/cf-conventions/cf-conventions-1.6/build/cf-conventions.html#units
>
> Thus, the absence of a unit is to be interpreted identically to a statement that
> units = '1'
>
> This is the current situation and it is likely that there is lots of data like this around.
>
> > Do we really need something more than a disambiguation of units = '1' vs no units attribute present?
>
> Yes, I think we do: this situation is not ambiguous in CF, they are the same thing.
>
> What I believe we require is a udunits entity which is clearly 'there is no unit of measure here, this is not dimensioned and not dimensionless'
>
> The udunits value
> ''
> delivers this functionality (I think), but it does not read very well, hence my suggestion that we ask for a new entry in udunits,
> 'no_unit'
> which is hopefully clear in its meaning and interpretation
> and which behaves the same as '' : failing all udunits processing attempts and operating as 'not a unit'
>
> all the best
> mark
>
> ________________________________
> From: CF-metadata [cf-metadata-bounces at cgd.ucar.edu] on behalf of Jim Biard [jbiard at cicsnc.org]
> Sent: 31 October 2014 15:18
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] string valued coordinates
>
> Mark,
>
> I'm not clear on what you are suggesting that udunits do with 'no_unit' or '?'.
>
> I had thought that the desire was to be able to differentiate between a pure number (as you mention below) and a value (whether a string or a bit pattern) that should not be interpreted as any number at all.
>
> As the situation stands, a units value of '1' is pretty unambiguously a marker for a pure number. We may need to modify docs to make this clearer, but I don't think that poses a problem. A variable with no units attribute at all is also pretty unambiguously a marker for something that isn't intended to be a even a pure number. Again, we may need to modify docs to make this clearer. Because these two concepts are somewhat conflated in the current documentation and usage (area_type being an example), there is the issue of other places where cleanup would be good going forward, but even if you have a units value of '1' on a non-number, it doesn't hurt anything in practice.
>
> Do we really need something more than a disambiguation of units = '1' vs no units attribute present?
>
> Grace and peace,
>
> Jim
>
> On 10/31/14, 11:04 AM, Hedley, Mark wrote:
> 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 [toyoda at gfd-dennou.org<mailto:toyoda at 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.graybeal at marinexplore.com<mailto:john.graybeal at 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.hedley at metoffice.gov.uk<mailto:mark.hedley at 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-bounces at cgd.ucar.edu<mailto:cf-metadata-bounces at cgd.ucar.edu>] on behalf of Jim Biard [jbiard at cicsnc.org<mailto:jbiard at cicsnc.org>]
> Sent: 30 October 2014 16:12
> To: cf-metadata at cgd.ucar.edu<mailto:cf-metadata at 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.hedley at metoffice.gov.uk<mailto:mark.hedley at metoffice.gov.uk>> wrote:
> Hello CF
>
> > 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>]
>
> > 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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Received on Wed Nov 05 2014 - 09:06:04 GMT
