Hi Martin and David, When I was writing about scalar coordinate variables in the CF convention, I considered a scalar to be a zero dimensional object. That is distinct from a 1 dimensional array which happens to only have 1 component. A coordinate array in the NUG sense is a one dimensional array. When a 1D array has just one component then the information content is like a scalar. But the terminology in the convention really refers to the data structure in the netcdf file.
Hope that helps, Brian On Wed, Dec 9, 2015 at 2:07 AM, <[email protected]> wrote: > Hi David, > > Aren't there two cases here, one in which the scalar coordinate does have > the same name as a dimension and one in which it doesn't? i.e. > > (1) scalar NUG coordinate variable > Dimensions: > dim1 = 1 ; > variables: > float myvar(dim1); > double dim1; > > (2) Scalar CF coordinate variable > variables: > float myvar; > myvar: coordinates= "dim1" ; > double dim1; > > I see that ticket 104 assumes that the term "scalar coordinate variable" > only refers to the 2nd example, but example (1) is declares a valid > coordinate variable in the NUG sense which is also a scalar. If CF wants to > exclude this, it needs to be explicitly stated that it is not allowed (or, > if it is already excluded by the convention somehow, this restriction > relative to the NUG convention should be clarified). > > I'm not sure that the reference to NUG is incorrect .. I certainly didn't > mean to assert that. I have the impression the NUG usage here is what > users expect and so it should be in the CF convention and the other parts > of the convention should be consistent. In what sense do you think it is > incorrect? > > Regards, > Martin > > -----Original Message----- > From: David Hassell [mailto:[email protected]] > Sent: 08 December 2015 14:19 > To: Juckes, Martin (STFC,RAL,RALSP) > Cc: [email protected] > Subject: Re: [CF-metadata] On scalar coordinate variables > > Hello Martin, > > I think that a CF scalar coordinate variable is not a NUG-defined > coordinate variable because it does not have the same name as a dimension. > > Nor is it a special type of CF coordinate variable, as was discussed in > ticket #104 http://cf-trac.llnl.gov/trac/ticket/104 - it could be > functionally equivalent to an auxiliary coordinate variable. > > However, section 1.3 makes it clear (in italics, no less) that > > "The use of [NUG-defined] coordinate variables is required for all > dimensions that correspond to one dimensional space or time > coordinates" > > which as you point out is incorrect. Perhaps that is where a clarification > should go, i.e.: > > "The use of coordinate variables or scalar coordinate variables (as > defined in section 5.7) is required for all dimensions that > correspond to one dimensional space or time coordinates" > > What do you think? > > All the best, > > David > > ---- Original message from [email protected] (09AM 08 Dec 15) > > > Date: Tue, 8 Dec 2015 09:58:29 +0000 > > From: [email protected] > > To: [email protected] > > Subject: [CF-metadata] On scalar coordinate variables > > > > Hello, > > > > The CF Convention 1.6 and draft 1.7 both include, in the discussion of > dimensions in Section 2.4, the statement that: > > "It is also acceptable to use a scalar coordinate variable which > eliminates the need for an associated size one dimension in the data > variable." > > > > However, the convention states that coordinate variables should be > interpreted as 'NUG-defined "coordinate variables."'. The NUG is vague > about the definition ( > https://www.unidata.ucar.edu/software/netcdf/docs/coordinate_variables.html > ), but it does say "Current application packages that make use of > coordinate variables commonly assume they are numeric vectors and strictly > monotonic". It also states that "A position along a dimension can be > specified using an index", which is not consistent with the use of a scalar > coordinate variable. > > > > One application which appears to assume that coordinate variables are > vectors is the CF Checker, so we need some clarification. I'm not sure how > other applications deal with it. > > > > The problem with the current phrasing in the CF Conventions document is > that it suggests the NUG approach is being followed and then introduces a > departure from the NUG approach in a separate part of the text. > > > > I would recommend either adding after 'NUG-defined "coordinate > variables"' a clarification '(that is a scalar or vector variable with the > same name as a dimension)', or changing the statement about use of scalar > coordinate variables. > > > > regards, > > Martin > > _______________________________________________ > > CF-metadata mailing list > > [email protected] > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > -- > David Hassell > National Centre for Atmospheric Science (NCAS) Department of Meteorology, > University of Reading, Earley Gate, PO Box 243, Reading RG6 6BB, U.K. > > Tel : +44 118 3785613 > E-mail: [email protected] > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >
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