[CF-metadata] Question about Coordinate System

From: Philip Bentley <philip.bentley>
Date: Wed, 07 Feb 2007 09:56:57 +0000

Hi Shawn,

I agree that this does appear to be a limitation of the current CF
specification. However, I have only been tracking this mailing list for
a few months, so I'm not in a position to say if this is a topic that
has previously come up for discussion. I'd be very surprised if it has
not surfaced before.

Based on a quick review of the CF 1.0 spec - and Appendix F in
particular - there may be a case for adding a new attribute called
'geodetic_datum' to the list of grid mapping attributes described in
Table F.1. The value of this attribute would then be a geodetic datum
identifier from a recognised dictionary or vocabulary, e.g. the widely-
adopted geodetic parameters database maintained by the European
Petroleum Survey Group (http://www.epsg.org/).

But there are side issues that would need to be discussed and agreed.
For example, should the value of this proposed new attribute be the
numeric identifier assigned by the dictionary maintainer (e.g. 6326 for
WGS 84), the well-known text identifier (e.g. "World Geodetic System
1984"), or either or both of these? The decision will be influenced by
the conflicting requirements for machine readability vs human
readability (i.e. how many people can interpret EPSG geodetic codes!?).

Also, there is the added complication of different types of datum, e.g.
vertical datum, engineering datum, and so on. While these are less
germane to the climate community, they may still need to be considered
if we want to arrive at a holistic solution. The work on modelling
coordinate reference systems being carried out under the aegis of ISO
TC211 and OGC (e.g. GML) should be able to inform this process.

Regards,

Phil

--
Phil Bentley PhD  CR Data Applications Manager
Met Office  Hadley Centre for Climate Change
FitzRoy Road  Exeter  EX1 3PB  United Kingdom
Tel: +44 (0)1392 884105  Fax: +44 (0)1392 885681
E-mail: philip,bentley at metoffice.gov.uk  http://www.metoffice.gov.uk
On Tue, 2007-02-06 at 20:04 +0100, Shawn Chen wrote:
> Hi, Listers.
> 
>  
> 
> I have a question of coordinate system after going through available
> documents. Correct me if I am wrong.  
> 
>  
> 
> Accodring to the CF-Convnetions, parameters such as projection
> approaches (eg. Lambert_Conformal_projection) are specified. However,
> I find no information about the DATUM.   Although it is assumed to be
> WGS84, there is possiblity for data in other datum such as
> Bessel-1841 specified in RT90. My question is, why not make the DATUM
> as a standard parameter? With all the effort being made by ncML, the
> inconsistency with other correctly geo-referencing dataset is still
> possible due to the lack of DATUM parameter. Am i naive or
> missing/misunderstanding something on the issue?
> 
>  
> 
> ps. some ideas about ncML and netCDF, please correct me if I am wrong
> One crucial point is that the ncML is the metadata of netCDF files.
> Information in ncML is orginated from netCDF. It's true that users can
> edit ncML to make it more complete, however, it still depends on the
> header of netCDF.   Suppose, a user trying to create a data in his
> national coordinate system (eg. RT90) using CF-Conventions.  The user
> specified the projections and relevant parameters. It seems fine at
> first. However, when the data is distributed and people who download
> the data can't be able to know that the datum is Bessel-1841 which
> specified in RT90. The system would transfer the coordinate system
> using WGS84 as datum.   That, eventually, would lead to inconsistency
> with other data.
> 
>  
> 
>  
> 
> Cheers
> 
> Shawn
> 
> 
> _______________________________________________
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> CF-metadata at cgd.ucar.edu
> http://www.cgd.ucar.edu/mailman/listinfo/cf-metadata
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Received on Wed Feb 07 2007 - 02:56:57 GMT