Hello Mark, I don't think the new text quite works, as it allows for the possibility of providing projection coordinates without either a grid mapping nor 2-d latitudes and longitudes; in which case the coordinate system is not sufficiently described.
If this proposal were to work, a grid mapping must be mandatory if the lats and lons were missing. I have understood the purpose of providing the lats and lons to be that calculating them on the fly from the projection coordinates and grid mapping parameters was too much to expect of the dataset user (rather than the dataset creator). Perhaps this is no longer the case? If so, we should note this. I think that this issue is not independent from `external_variables` - allowing auxiliary coordinates to be external might, in some circumstances, allow the 2-d lats and lons to remain mandatory whilst addressing the file size issue. -- 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/pull/133#issuecomment-399911850
