Dear all Before the meeting yesterday I was arguing, like Karl @taylor13, to retain the presumption that if the `grid_mapping` and `crs_wkt` are inconsistent, the data-user should presume the `grid_mapping` is correct. I was persuaded by the discussion that this isn't generally helpful, because the data-user might well think it was unsafe to proceed on that basis. In particular, the data-user might be aware that the WKT information had come first, and therefore suspect the `grid_mapping` of being an incorrect translation. Karl and I had argued also that the presumption of `grid_mapping` being correct gives an extra incentive to the data-producer to make sure the two are consistent. However, this also probably doesn't work; if the data-producer had thought about the consequence of misinterpretation, they would have tried to avoid inconsistency anyway.
Therefore I support the change to remove this assumption, and state that the metadata is invalid if `grid_mapping` and `crs_wkt` are inconsistent. In response to Karl, I agree with Alan @snowman2 that this is an error, and the file is not compliant, because the convention states the two kinds of metadata must be consistent. Unfortunately, the CF checker won't be able to detect this error unless we write down the mapping between `grid_mapping` attributes and `crs_wkt` in the conformance document (or some document it can refer to). To make the check, you have to be able to interpret both kinds and compare them. One of Alan's concerns was that data-users felt they had to do that. We agreed that they don't. They can read one or the other, assuming they agree. However, it would be good if this could be checked routinely. As I argued before, I strongly feel that it would improve the convention if we could write down the equivalence. Note that we don't have to consider _all_ aspects of WKT, but only those aspects which people want to write in CF-netCDF files. Doing this would cause us to identify what has to be added to `grid_mapping` to give it the required capabilities, and whether there are inconsistencies between the CF data model and WKT. I wouldn't be surprised if there are, and we need to know, because it's not safe to treat `crs_wkt` as a black box if it might conflict with other CF metadata (not just in the `grid_mapping`, but for instance in the units and standard name of coordinate variables). I would be concerned about adopting Philip @cameronsmith1's suggestion, because I fear that might lead to data-producers not being so careful with one or the other of the representations, thinking that they could set the flag to indicate it's not to be trusted. Jonathan -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: https://github.com/cf-convention/cf-conventions/issues/222#issuecomment-643267858 This list forwards relevant notifications from Github. It is distinct from [email protected], although if you do nothing, a subscription to the UCAR list will result in a subscription to this list. To unsubscribe from this list only, send a message to [email protected].
