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 <chen14742@xxxxxx>
Reply-To: Hsiang Yun Chen <chen14742@xxxxxx>
Organization: UCAR/Unidata
To: <netcdfgroup@xxxxxxxxxxxxxxxx>

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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