Re: [Fwd: Question about Coordinate System]

Hi Shawn:

John Caron wrote:
> -------- Original Message --------
> Subject: Question about Coordinate System
> Date: Tue, 6 Feb 2007 16:03:36 +0100
> From: Hsiang Yun Chen <address@hidden>
> Reply-To: Hsiang Yun Chen <address@hidden>
> Organization: UCAR/Unidata
> To: <address@hidden>
>
> 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?

this is definitely a shortcoming in CF-Conventions. Would you like to post your 
comments to that mailling list? you have to join first: 
http://www.cgd.ucar.edu/mailman/listinfo/cf-metadata

> 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.


You can add/change metadata using NcML. The Java library can read the NcML 
file, which points to the original netcdf file. The C library will eventually 
be able to do this also. To distribute the modifications as a netcdf file, you 
will have to rewrite the netcdf file, which can be easily done with the Java 
library.

John

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