On 7/26/2011 4:24 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.
>
> 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
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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.
Received on Wed Aug 10 2011 - 06:51:49 BST
