> 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.
I hope to present on this very issue at AGU. The problem we see with ambiguous definition of datums is a cascade of non-recognition of datums through processing algorithms and in the output of some processes that generate very detailed data.
The prime example is downscaled climate data. Because the climate modelers involved generally consider lat/lon to be a lowest common denominator, the datum used to geolocate historical data (like rain gages) is neglected. What results is, in our case, a 1/8deg (12km) grid with no datum. This is unacceptable. As at this resolution, the errors in a wrong assumption of datum for the grid can cause very substantial (a full grid cell or more) geolocation errors.
If the CF community intends to consume any ground based data, then datums must be preserved from ingest of ground based forcing throughout data storage and processing. This is fundamental information that is required for ALL data comparison operations.
I would argue that CF compliance should require this information. This puts the requirement to make metadata assumptions on data publishers/producers rather than data consumers. It is unacceptable to have different data consumers making different assumptions of geolocation on the same data.
Off soapbox.
Dave Blodgett
Center for Integrated Data Analytics (CIDA)
USGS WI Water Science Center
8505 Research Way Middleton WI 53562
608-821-3899 | 608-628-5855 (cell)
http://cida.usgs.gov
On Jul 26, 2011, at 5:24 AM, Jonathan Gregory wrote:
> Dear all
>
> For 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.
>
> Cheers
>
> Jonathan
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