[CF-metadata] the need to store lat/lon coordinates in a CF metadata compliant netCDF file

From: Jonathan Gregory <j.m.gregory>
Date: Tue, 26 Jul 2011 11:24:46 +0100

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
Received on Tue Jul 26 2011 - 04:24:46 BST