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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 > What this discussion has helped clarify for me is that coordinate variables are independent, and auxiliary coordinates are dependent variables. I think thats a really valuable advance in the data model. I agree. We will put that in the data model. > OTOH, theres nothing special about a scalar coordinate, and should not be handled in a special way in the data model. Its just an auxiliary coordinate, period. By the current definition of coordinate and auxiliary coordinate, its clearly an auxiliary coordinate. It is formally an auxiliary coordinate variable, but not logically so. That's a point that Steve made and is included in the motivation for this ticket - now a long way back in [https://cf- pcmdi.llnl.gov/trac/ticket/104#comment:28 comment 28]: Scalar coordinate variables provide a convenient way to encode coordinate variables of size one. They do so by borrowing the syntax that is otherwise used for auxiliary coordinate variables. There is, however, a key difference between the interpretation of scalar coordinate variables and auxiliary coordinate variables. Scalar coordinates have the same status in a CF file as (conventional, Unidata, COARDS) coordinates in which the dimension name and the variable name match. These coordinates define the independent variables (spatiotemporal and others) for the data variable. Auxiliary coordinate variables provide extra information as a function of these independent variables, as alternative numeric values (which don't have to be unique or monotonic along a given dimension), or string-valued labels. To indicate that a variable is intended to be an auxiliary coordinate variable, it is necessary to give it a dimension, in order to show which coordinate variable(s) it belongs to. Numeric scalar coordinate variables are not to be interpreted as auxiliary coordinate variables. Yes, the single timeseries representation with scalar `lat` and `lon` coordinates would, according to this ticket, be regarded as having independent dimensions of lat and lon. That is a possible interpretation, and I would say it's the preferred one in the case that the timeseries was extracted from a gridded dataset. I would argue that it ''is'' possible, according to this ticket, to store the data with the interpretation you prefer, that it's one timeseries from a DSG. That is what it means if we use auxiliary coordinates i.e. {{{ dimensions: station=1; time=NNN; variables: float lat(station); float lon(station); float time(time); float temp(station,time); temp:coordinates="lat lon"; }}} In this case, the `station` dimension is a discrete axis. CF section 4.5 says a discrete axis "indicates either an ordered list or an unordered collection, and does not correspond to any continuous coordinate variable." So it is a netCDF dimension, but it doesn't correspond to any independent physical dimension. It's just an index; only `time` is an independent physical dimension. Isn't that what you want? Consider the case with `station=2` but otherwise the same. Now you have two 1D timeseries, each with a single independent physical dimension of time. The other netCDF dimension is just an index to bundle them up. You could put exactly the same data in a one of the chapter 9 ragged representations. Then the data array would have only one netCDF dimension, but the station dimension would still exist, and you would still have `lat(station)` and `lon(station)` just as above. Again, the `station` dimension is simply an index. Would you agree that this does not add any physical dimensions to the individual timeseries? Best wishes Jonathan -- Ticket URL: <https://cf-pcmdi.llnl.gov/trac/ticket/104#comment:59> 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.
