projections for the CF standard

• To: netcdfgroup@xxxxxxxxxxxxxxxx
• Subject: projections for the CF standard
• From: Jonathan Gregory <jonathan.gregory@xxxxxxxxxxxxx>
• Date: Wed, 19 Feb 2003 13:50:54 +0000

Dear All

We are currently designing attributes in the CF netCDF standard to define
grid mappings. By a "grid mapping" we mean a continuous transformation which
maps one 2D (horizontal) surface to another. Some of them are projections of
the Earth's surface onto a plane (map projections), and others are trans-
formations of the spherical surface. In CF, variables defined on a horizontal
surface are required to supply latitude and longitude for all the points
explicitly, for the sake of applications which don't understand grid mappings.
However, if the 1D coordinate variables are not latitude and longitude
(for instance, projection x- and y-coordinates), it is helpful to supply
the grid mapping which will convert the coordinate variables to latitude and
longitude, for use by applications which understand the mapping.

The following is a list of grid mappings we are aware of being used in GCMs,
RCMs and datasets with which such models are compared. We propose to include
these in the CF standard.

Lambert conformal
conformal cubic
transverse Mercator
polar stereographic
rotated pole
stretched grid (as in the Arpege model)

Are there others which should be considered? The CF standard will continue to
evolve so this list does not have to be exhaustive now. We want to include
those which are actually in use by people who might use CF.

Thanks for your help.

Jonathan Gregory  jonathan.gregory@xxxxxxxxxxxxx
phone +44 1344 854542  fax +44 1344 854898
Hadley Centre, Met Office, London Road, Bracknell, Berks., UK. RG12 2SY