On 7/26/2011 4:24 AM, Jonathan Gregory wrote:
Dear allFor datasets which are intended for analysis by end-users I think it would be undesirable to remove the requirement of providing explicit lat and lon coords even if a grid_mapping is provided. I think it is unrealistic to expect all software which someone might use to analyse netCDF files to be able to recognise and act upon all possible values of the CF grid_mapping attribute, and without the lat and lon information the user would have a problem. If the issue is storage space in the file I think the much better choice is to store the explicit coordinates in another file, by extending the CF convention to allow datasets to be distributed over several linked files, as gridspec does for example. Steve appears to suggest that grid_mapping is required in some circumstances, but I don't think it is at present. However, the text Steve quotes may not be quite right: "/When the coordinate variables for a horizontal grid are not longitude and latitude,*_it is required that the true latitude and longitude coordinates be supplied_* via the coordinates attribute/." The text should make it clear that this requirement applies when the data has a geolocated horizontal grid. It doesn't necessarily apply to idealised cases. We could clarify this with a defect ticket. Without the grid_mapping, the lat and lon still make sense in the common case (and original CF case) of GCM data, and in many other cases, the intended usage of the data does not require precision about the figure of the Earth. Although this metadata could be valuable if it can be defined, I think it would be too onerous to require it. CF is primarily intended "to promote the processing and sharing" of netCDF files, it says in the Abstract. As John notes, it could be used for other purposes, such as internal files in models. Do they have to be CF-compliant, however, if they're not intended for analysis or for sharing? Cheers Jonathan _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
Storing coordinates in a separate file is a reasonable solution for large, managed archives, not so good for data exchange (eg extract a subset from large archive and give it to ArcGIS to read). I think we should provide a profile of CF which relaxes the requirement for 2D lat/lon coordinates, as long as grid_mapping is supplied.
We also may need a profile of CF for GIS data, eg to make the datum be required. Then we could say "file is CF/GIS compliant" without adding unneeded requirements for climate modelers.
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