Dear all,
Here's what the CF conventions say about dimensionless quantities:
"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|. "
It does not indicate whether or not it is o.k. to set units to the empty
string, but by *not* mentioning this, I think we should assume that this
would not be CF compatible. I also think that a human interpreting
units = "" might think that the user had made mistake, whereas if the
units attribute were missing altogether, they would think it was simply
unitless. A strict interpretation of the conventions seems to imply:
1. a number (like "1" or "1e-6") should be used if the quantity
represents fractions or parts of a whole, whereas
2. the units attribute should be omitted in other dimensionless cases
(e.g., involving ratios such as calcium/boron ratio, or involving
"counts" such as the number of cars crossing a bridge each day) In
these cases the standard name indicates what the quantity is (but I
wonder what standard name applies in the second example above, and
should it have units of inverse time?)
If a quantity is dimensionless and the units attribute is present, I
think the CF checker currently only allows "1" as the units attribute.
Perhaps this should be generalized since apparently a conforming unit is
also a number (such as "1e-6" for ppm). I don't think it should allow
units = "".
Best regards,
Karl
On 3/1/11 9:26 AM, Steven Emmerson wrote:
> Everyone,
>
> The UDUNITS packages (both UDUNITS and UDUNITS-2) interpret the empty
> string (i.e., "") as the dimensionless unit 1 -- so if you can't use
> "1", then use "" (assuming you pass that string to the UDUNITS library).
>
> The udunits(1) and udunits2(1) programs, however, interpret the empty
> string as a user input error and ask for the input unit again.
>
> Regards,
> Steve Emmerson
> UDUNITS developer
>
> On 2/28/2011 10:18 AM, Rosalyn Hatcher wrote:
>> Dear Karl,
>>
>> That is correct. If the variable is deemed unitless, then the checker
>> will not flag an error if either units=1 or the units attribute is omitted.
>> If the units attribute is missing, it will, however, produce an
>> information message suggesting that the units attribute is added for
>> completeness.
>>
>> Eg.
>>
>> ------------------
>> Checking variable: CHL_count
>> ------------------
>> INFO (3.1): No units attribute set. Please consider adding a units attribute for completeness.
>>
>>
>> I introduced the INFO category of message a while ago, for instances
>> where the checker couldn't be sure of an error (E.g. where an attribute
>> is being used in a non-standard way), to prompt the user to double check
>> that what they've put is actually what they meant.
>>
>> I'll hopefully have a fixed version of the checker available tomorrow
>> for testing.
>>
>> Regards,
>> Ros.
>>
>>
>>
>>
>> On 28/02/11 16:44, Karl Taylor wrote:
>>> Dear Rosalyn,
>>>
>>> I assume the checker will also not complain if the units attribute is
>>> simply omitted when the variable is unitless (i.e., either units="1"
>>> or the attribute is omitted result in the same behavior by the checker).
>>>
>>> best regards,
>>> Karl
>>>
>>> On 2/28/11 4:28 AM, cristina.tronconi at artov.isac.cnr.it wrote:
>>>> Dear Rosalyn,
>>>> thanks so much for your reply and your works that is so important for
>>>> me. Infact "my" data are produced within MYOCEAN (european) project
>>>> and thay are going to test them! so it is important they do not failed
>>>> the CF checker test.
>>>>
>>>> Best regards.
>>>>
>>>> cristina
>>>>
>>>> p.s.
>>>> Sorry if sometime I miss some e-mails and I didin't answer to them.
>>>> This is a very caotic working time....and I'm receiving so many e-mails.
>>>>
>>>> Thanks to all the cf-metadata group to their help!
>>>>
>>>> Citando Rosalyn Hatcher<r.s.hatcher at reading.ac.uk>:
>>>>
>>>>> I agree units should be set to "1" and I am currently in the process of
>>>>> fixing the error in the CF Checker.
>>>>>
>>>>> Regards,
>>>>> Ros.
>>>>>
>>>>>
>>>>> On 27/02/11 17:27, Steven Emmerson wrote:
>>>>>> Cristina,
>>>>>>
>>>>>> I recommend the unit "1" for that use.
>>>>>>
>>>>>> If the CF checker doesn't like that unit, then it should be fixed,
>>>>>> IMO, because that unit is supported by both the US NIST and the BIPM.
>>>>>>
>>>>>> Regards,
>>>>>> Steve Emmerson
>>>>>> UDUNITS developer
>>>>>>
>>>>>> On 2/26/2011 12:30 PM,cristina.tronconi at artov.isac.cnr.it wrote:
>>>>>>> Dear Roy,
>>>>>>> Our variable "chlorophyll count" as is understandable by CF is the
>>>>>>> number of individual measurements used to determine a chlorophyll value.
>>>>>>>
>>>>>>> I do need to know what to put at "units" attribute for that variable.
>>>>>>> I undestood by the CF convention 1.4 (appendix C) that I have to put
>>>>>>> "units="1" as the chlorophyll count is a dimensionless variable...but
>>>>>>> the CF checker found it as an error.
>>>>>>>
>>>>>>> I try not to put the Unit attribute ...but again i get an error by che
>>>>>>> cf chcker.......so.....
>>>>>>>
>>>>>>> my question is: what unit shoudl I set for my chlorophyll count variable?
>>>>>>>
>>>>>>> thanks for your help.
>>>>>>>
>>>>>>> cristina
>>>>>>>
>>>>>>> -------------------------------------------------------------------
>>>>>>> Cristina Tronconi
>>>>>>> Istituto di Scienze dell'Atmosfera e del Clima - sezione di Roma
>>>>>>> Consiglio Nazionale delle Ricerche
>>>>>>> Via Fosso del Cavaliere 100
>>>>>>> 00133 Roma, Italy
>>>>>>> Tel: +39 06 49934342
>>>>>>> cell: '39 349 1242954
>>>>>>> Fax: +39 06 20660291
>>>>>>> e-mail:cristina.tronconi at artov.isac.cnr.it
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> CF-metadata mailing list
>>>>>>> CF-metadata at cgd.ucar.edu
>>>>>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>>>>> _______________________________________________
>>>>>> CF-metadata mailing list
>>>>>> CF-metadata at cgd.ucar.edu
>>>>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>>>> --
>>>>> Rosalyn Hatcher
>>>>> NCAS Computational Modelling Services
>>>>> Dept. of Meteorology, University of Reading,
>>>>> Earley Gate, Reading. RG6 6BB
>>>>> Email:r.s.hatcher at reading.ac.uk Tel: +44 (0) 118 378 6016
>>>> -------------------------------------------------------------------
>>>> Cristina Tronconi
>>>> Istituto di Scienze dell'Atmosfera e del Clima - sezione di Roma
>>>> Consiglio Nazionale delle Ricerche
>>>> Via Fosso del Cavaliere 100
>>>> 00133 Roma, Italy
>>>> Tel: +39 06 49934342
>>>> cell: '39 349 1242954
>>>> Fax: +39 06 20660291
>>>> e-mail:cristina.tronconi at artov.isac.cnr.it
>>>>
>>>>
>>>> _______________________________________________
>>>> CF-metadata mailing list
>>>> CF-metadata at cgd.ucar.edu
>>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>>
>>> _______________________________________________
>>> CF-metadata mailing list
>>> CF-metadata at cgd.ucar.edu
>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>
>> --
>> Rosalyn Hatcher
>> NCAS Computational Modelling Services
>> Dept. of Meteorology, University of Reading,
>> Earley Gate, Reading. RG6 6BB
>> Email:r.s.hatcher at reading.ac.uk Tel: +44 (0) 118 378 6016
>>
>>
>>
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>> CF-metadata at cgd.ucar.edu
>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
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