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#104: Clarify the interpretation of scalar coordinate variables -----------------------------+---------------------------------------------- Reporter: jonathan | Owner: [email protected] Type: enhancement | Status: new Priority: medium | Milestone: Component: cf-conventions | Version: Resolution: | Keywords: -----------------------------+---------------------------------------------- Comment (by jonathan): Dear John, Karl, Jim I think Jim is correct in saying that you can have a number of 1D auxiliary coordinate variables of the same dimension, without there being a (Unidata, COARDS) coordinate variable of that dimension. This might well happen, for example, with a model ensemble axis. The ordering of such an axis is arbitrary, and there isn't necessarily a monotonic coordinate variable for it. The elements are distinguished by a combination of information such as ensemble member number and model identity. Therefore I don't think we can prescribe whether coordinate or auxiliary coordinate variables should be used. I agree with John to the extent that coordinate variables and auxiliary coordinate variables are similar and sometimes exchangeable semantically, but I wouldn't say they are the same. The differences that we state in the draft data model ([https://cf-pcmdi.llnl.gov/trac/ticket/95#comment:52 latest version of this text]) are that coordinate variables must be one- dimensional, numeric and monotonic (implying all their values must be distinct). Auxiliary coordinate variables can be multidimensional, string- valued and don't have to be monotonic. Note that in the data model we use the term ''dimension coordinate construct'' to correspond to a (Unidata, COARDS) coordinate variable, and ''auxiliary coordinate construct'' for a (CF) auxiliary coordinate variable. To address Karl's other concern, I would propose If a data variable has two or more scalar coordinate variables, they are regarded as though they were all independent coordinate variables with dimensions of size one. If two or more single-valued coordinates are not independent, but have related values (this might be the case, for instance, for time and forecast period, or vertical coordinate and model level number, Section 6.2), they should be stored as coordinate or auxiliary coordinate variables of the same size one dimension, not as scalar coordinate variables. That's only slightly different from before. I've inserted "this might be the case", because time and forecast period ''might'' be related, but they might not, as Karl suggests - it depends on the application. It's valuable to indicate whether there is a relationship or not (this has been discussed earlier in this ticket). Is this text OK? Best wishes Jonathan -- Ticket URL: <https://cf-pcmdi.llnl.gov/trac/ticket/104#comment:36> CF Metadata <http://cf-pcmdi.llnl.gov/> CF Metadata This message came from the CF Trac system. To unsubscribe, without unsubscribing to the regular cf-metadata list, send a message to "[email protected]" with "unsubscribe cf-metadata" in the body of your message.
