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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 If you have a coordinate variable `lat(time)`, I think it must be a trajectory - is that right? In Table 9.1, we define a trajectory as "a series of data points along a path through space with monotonically increasing times", so it is natural to record time as the independent coordinate, and latitude depends on it. In that situation, according to this ticket, you would ''not'' replace `lat(time)` with a scalar `lat`, even if the latitude were unchanging all the way along the trajectory, because doing so would incorrectly imply that latitude is independent. You have to keep `lat(time)` and fill it with repeated values. Actually, I think that is better anyway in this case, precisely because latitude really is a function of time. I think it would be strange to use a different way of storing a trajectory if the latitude values were [51, 51, 51, 51] on the one hand, or [51, 51.001, 51, 51] on the other. For timeseries data, the latitude and time coordinates are independent. If it's a single timeseries, it will have only one latitude, and in that situation, according to this ticket, it would be fine to replace the size- one coordinate variable `lat(lat)` with the scalar `lat`, and omit the dimension `lat=1`. If you have data on multiple vertical levels with a physical coordinate, it might have dimensions `data(pressure,lat,lon)`. When you extract a single level, you reduce the dimension to `pressure=1`. The size-one `pressure(pressure)` is an independent coordinate. In this situation, CF permits you to replace it with a scalar `pressure` and omit the size-one dimension. You don't have to do this; it's a convenience feature, as the standard document says. According to this ticket, replacing `pressure(pressure)` with `pressure` does not alter the interpretation. It is still a size-one independent coordinate. This ticket says that the scalar coordinate implies the existence of a size-one dimension. I agree that you do not add a vertical dimension for temperature at the tropopause, because tropopause doesn't have a numerical coordinate to define it. CF provides standard_names for quantities that exist on special physically defined leves, for this reason. CF does not provide standard names for levels which can be specified with coordinate values. There isn't a standard name for temperature at 2.0 m height, because that can be specified with a size-one `height` coordinate. It could be either `height(height)` with dimension `height=1`, or a scalar `height`. According to this ticket, they both logically describe an independent size-one dimension. CF has this convenience feature because it allows you to have many size- one independent coordinates without having to create netCDF dimensions for them. This ticket says that the domain logically has these size-one dimensions anyway. Is that all right? Cheers Jonathan -- Ticket URL: <https://cf-pcmdi.llnl.gov/trac/ticket/104#comment:48> 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.
