[CF-metadata] Question on WKT representation of CRS

From: Jonathan Gregory <j.m.gregory>
Date: Fri, 30 Dec 2011 13:06:21 +0000

Dear Etienne

Thanks for your posting of 13 Dec and apologies again for slow reply.

> => In light of all this, could we add datum_name, datum_code and
> towgs84 to grid_mapping?

I think we should call it horizontal_datum_name (since there are also vertical
datums). We also need various other names to store the strings which are used
in WKT: the projected coordinate system, geographic coordinate system,
reference ellipsoid and prime meridian all have names in your WKT strings.
If these names are valuable, we should define attributes for them too. Are
these names standardised, or are they just user-readable information (like
netCDF long_names)?

Yes, we need towgs84, and also prime_meridian_longitude (as well as its name).

My only reservation is about including the code and authority in grid_mapping.
I am concerned about this because it is redundant information if the mapping
is fully described by the other attributes. There is a danger that it might be
inconsistent with those attributes e.g. the mapping might claim to be EPSG
4618 but differ from the proper definition of that mapping in some way. Then
applications would behave differently according to which definition they
followed. This is the same kind of concern as I have (in ticket 69) about the
possible inconsistency of WKT and grid_mapping if both are stored in the file.

> As maintainer of GDAL's netcdf driver, I see that the only thing
> missing in the WKT/CF transformation is named datum (and ideally datum
> code and towgs84), then there would not be any need to keep the entire
> WKT string

That would be the best solution, in my opinion. I am encouraged by this.

> with some exceptions where the parameters are not 100% compatible

I hope that we will be able to resolve these differences, as detailed in your
exchanges with John. Have you yet been able to obtain a trac username, so that
you can open a ticket to correct defects in grid_mapping definition and propose
new ones?

> > The VERT_CS and VERT_DATUM appear to be names (in the Newlyn example). Are
> > these names standardised?
>
> They are part of CT 1.0 , I suggest you take a look at the WKT
> keywords.

The definition of the WKT keywords isn't helpful, unfortunately. The document
linked to the CT website doesn't say anything I can find about what values they
can take. I suppose we should postpone consideration of vertical reference
systems until someone comes forward with a specific use case. In fact, that
information might belong better with the CF vertical coordinate, like
formula_terms, rather than in the grid_mapping.

Best wishes and happy 2012

Jonathan
Received on Fri Dec 30 2011 - 06:06:21 GMT