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
