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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 Yes, we agree, this whole ticket is really about clarifying the data model. The clarifications being discussed are concerned with how CF-netCDF metadata are interpreted, and do not affect the legality of CF-netCDF files, although they do imply that some ways are better than others for encoding a given dataset. The part you quote last is the crucial issue in this debate. I think you have made a good point, and thanks for that. In the [https ://cf-pcmdi.llnl.gov/trac/ticket/95#comment:52 draft data model as it stands], we distinguish dimension (Unidata, COARDS) coordinates and auxiliary coordinates purely on formal grounds (uniqueness, monotonicity, dimensionality, data type). That's because we based the data model on the CF-netCDF convention, of course. But I think you have correctly identified the conceptual distinction, and we should put that in the data model document as well, namely that the dimension coordinate variables are independent, and the auxiliary coordinate variables are dependent. It's because the dimension coordinates are independent that they must be unique and one-dimensional. Those formal properties don't help with a scalar coordinate, as you say, but the idea that it's an independent variable is still valid. In some situations, a CF-netCDF file might have auxiliary coordinate variables of dimensions which do not have dimension coordinate variables. One situation is an axis for an unordered collection, such as an ensemble axis. In that case, I suppose the index along the ensemble dimension is the independent variable, in a sense, but that is not useful information since the ordering is arbitrary, and there's no need for explicit independent coordinates. In the case you mentioned earlier, of 2D lat and lon auxiliary coordinate variables if the 1D projection coordinates are not given, I would say that the auxiliary coordinates are still dependent on the projection coordinates. Even though the latter are absent, there are formulae which define the relationship. Will this ticket be OK, do you think, if we add some text to insert a couple of sentences in the CF-netCDF standard, when the two kinds of coordinate variable are introduced, to point out their distinction in role of independence/dependence? If so, I'll draft some extra text for this ticket. Do you think we really need to ''define'' what independence and dependence mean in the CF-netCDF standard, or can we assume that people will understand them in their usual mathematical sense? Cheers Jonathan -- Ticket URL: <https://cf-pcmdi.llnl.gov/trac/ticket/104#comment:45> 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.
