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#104: Clarify the interpretation of scalar coordinate variables -----------------------------+---------------------------------------------- Reporter: jonathan | Owner: [email protected] Type: defect | Status: new Priority: medium | Milestone: Component: cf-conventions | Version: Resolution: | Keywords: -----------------------------+---------------------------------------------- Comment (by markh): I will try to illustrate a useful example. Consider a CF compliant NetCDF data variable with a 2 dimensional data array and two coordinate variables: * latitude * longitude This data variable declares auxiliary coordinates, using the coordinates attribute. All but one of these are scalar coordinate variables: * surface_pressure(latitude, longitude) * a * p0 * b * time * forecast_period * forecast_reference_time * source * experiment_id * model_id We have many such data sets. Often these are converted from other formats using one of a number on converters developed inside my office and by collaborating organisations over the past few years. Sometimes they are the results of analyses which have been persisted by researchers. These have always been viewed as reasonable data sets, given the comprehension of scalar coordinate variables. Under this proposal, this data set would be viewed as exactly equivalent to a CF compliant NetCDF data variable with an 11 dimensional data array and 11 coordinate variables: * latitude * longitude * a * p0 * b * time * forecast_period * forecast_reference_time * source * experiment_id * model_id along with one auxiliary coordinate: * surface_pressure(latitude, longitude) The proposal makes explicit that there should be no semantic differentiation between the two data sets, the first is merely an encoding short hand for the second. This interpretation does not adequately reflect the data and metadata. It defines a set of independent coordinates which were not intended to be independent by the data creator. This will cause considerable confusion in the interpretation of these data sets, most of which are in storage, to be returned to at a later date as required. If the data creators had been shown an 11 dimensional data set when they created it, they would have very likely raised questions about the output, but the scalar coordinate variable has not been interpreted in this way. The first encoding appeared to people to be a logical and sensible representation of their data. Scalar coordinate variables have been viewed by users as semantic containers: no clear statement existed in the conventions to disagree with this perspective. In most cases these data creators used software which had been deemed appropriate by the organisation and trusted its output would be valid. For years it has been deemed to be valid. With cases such as this in mind, I do not think the community have the luxury of making decisions solely based on confidence in what the authors of the convention had in mind when scalar coordinate variables were invented. I think the interpretation of these features of the convention by users needs to be taken into account. I have previously posted examples from the discrete sampling geometries section of the conventions which require different interpretation. I do not agree with the proposed approach of handling these as further special cases, requiring further clarification text not in this proposal. -- Ticket URL: <https://cf-pcmdi.llnl.gov/trac/ticket/104#comment:13> 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.
