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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 caron): Hi Jonathan and all: Well, if we had to make a decision here, I would say that "coordinate variables" = "independent" and "auxiliary coordinates" = "dependent" would be the correct one. Obviously very simple and powerful idea. For all the examples I can think of, this interpretation seems reasonable to me. It would be good if others look at their data files and see if there is an important exception to this rule. For gridded data, I agree that aux coordinates lat(x,y) and lon(x,y) are best thought of as dependent on coordinate variables x(x) and y(y), even when x(x) and y(y) are missing. For point data, I agree that aux coordinates lat(time), lon(time) are best thought of as dependent on the coordinate variable time(time). However, under this interpretation, it seems that a scalar auxiliary coordinate should be thought of as dependent, not independent. For example if "lat(time)" is actually constant, so one uses a scalar auxiliary coordinate "lat" in its place, it seems that it is still a dependent variable, and should not be thought of as adding another degree of freedom to the data, ie with some implicit dimension lat(1). Conversely, if you want to indicate that the domain has another independent coordinate, you should dimension your data with it, even if that dimension happens to only be of length 1, and you should add a coordinate variable of length 1. A good example I run across a lot in GRIB data are vertical coordinates. If one could have multiple vertical levels, say on pressure levels, I will add a vertical dimension even if theres only one such level in the file. But for things like "Temperature at the tropopause" I wont add a vertical dimension. Obviously one could do it differently, but I think thats reasonable. This interpretation has the advantage that one can add as many auxiliary coordinates as you need, without increasing the dimensionality of the domain. I think that's the essence of what it means to have n independent coordinates. What do you think? John Regards, John Replying to [comment:45 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:46> 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.
