> 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 [[email protected]] on behalf of Jim Biard [[email protected]] Sent: 30 October 2014 16:12 To: [email protected] 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 <[email protected]<mailto:[email protected]>> wrote: Hello CF > From: CF-metadata > [[email protected]<mailto:[email protected]>] > on behalf of Jonathan Gregory > [[email protected]<mailto:[email protected]>] > 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 _______________________________________________ CF-metadata mailing list [email protected]<mailto:[email protected]> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected]<mailto:[email protected]> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata -- [CICS-NC] <http://www.cicsnc.org/> Visit us on Facebook <http://www.facebook.com/cicsnc> Jim Biard Research Scholar Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/> North Carolina State University <http://ncsu.edu/> NOAA's National Climatic Data Center <http://ncdc.noaa.gov/> 151 Patton Ave, Asheville, NC 28801 e: [email protected]<mailto:[email protected]> o: +1 828 271 4900
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