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Dear John I completely sympathise with encouraging people to provide useful metadata. I would argue against a mandatory Z (or any) coordinate, though, as the data really might not have a useful Z coordinate. I'm guessing, but what about the sea state (its roughness due to waves)? That's a property of the sea surface one might record, but it doesn't have a height, and the ship observing it is obviously at sea level. Or the total cloud amount, say? This also doesn't relate to a particular altitude in the atmosphere. You could record the altitude of the station that observed it, but I would not say that was really a coordinate of the cloudiness itself, and again it would not be useful for marine obs. Requiring instead an "unknown" indicator or a missing data indicator strikes me as excessive, and I'm not convinced data-writers would always do it, or that it would be useful if they did. There are lots of features of CF metadata which are very valuable but optional, such as bounds, cell_methods and standard_names. Cheers Jonathan