Dear Jonathan,
Thank you for clear this up.
>When it has been discussed before, the point has been made that
>this information doesn't exist for most models e.g. global climate models,
>which treat the Earth as a sphere, so it would be optional (like many other
>features of CF).
Please correct me if I am wrong. Isn't the definition of the ellipsoid and
prime meridian still necessary for data distribution in order to avoid
confusion among users who didn't create the dataset?
>It would help me/us to understand it if you could describe the required
>information in more detail and what it is used for.
I am not familiar with the usage of the dataset therefore I am in no
position to describe the require information. I believe that only data
users can give a good description in terms of which information is
necessary. However, I can explain what it is used for.
I raised the issue due to interoperability between earth science and
geoinformation realms. There were intense cooperation in GALEON IE projects
and a THREDDS-WCS gateway is implemented as the solution to distribute
climate/marine data to GIS community. With the amazing jobs that have been
conducted, it would be a shame if GIS users encounter problems in terms of
geo reference using netCDF data that is supposed to contains all the
necessary information. My impression is the lack of DATUM description may
be work fine for professionals in atmosphere and oceanography realms as you
chose to make it optional. Nevertheless, it could be a crucial component for
interoperability. In order to make things right and the efforts worthwhile,
I sincerely look forward to a standard that resolve the ambiguity in the
near future.
On 08/02/07, Jonathan Gregory <j.m.gregory at reading.ac.uk > wrote:
>
> Dear Shawn and Phil
>
> This issue has indeed come up before more than once, but not been
> resolved.
> It would help me/us to understand it if you could describe the required
> information in more detail and what it is used for. Is a "datum" such as
> WGS84
> a definition of a geometrical surface (ellipsoid) that is used to
> approximate
> the geoid (a surface of constant geopotential)? If so, then it is relevant
> both for making more precise what we mean by quantities which are measured
> with
> respect to the geoid, such as sea surface height, and as input to
> conversions
> between lat-lon and map projection coordinates (the latter being why you
> raised it). When it has been discussed before, the point has been made
> that
> this information doesn't exist for most models e.g. global climate models,
>
> which treat the Earth as a sphere, so it would be optional (like many
> other
> features of CF).
>
> > 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 self-describing principle of CF would indicate that we would use a
> text
> identifier. It could have a numeric component.
>
> It would be good to make use of a dictionary maintained by someone else,
> like
> EPSG, but this depends on their dictionary having exactly the right sort
> of
> contents for us and available in an appropriate format. We have not
> managed to
> do that in other similar cases. Instead we have adapted other sources of
> information and imported them into CF. This introduces issues of
> maintenance,
> but on the other hand it makes CF more robust.
>
> Possibly this discussion is related to the one raised by Bert Jagers in
> the
> autumn (see http://cf-pcmdi.llnl.gov/trac/wiki/MultipleProjections ).
>
> Best wishes
>
> Jonathan
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--
I have a thing about first impressions.
--
I have a thing about first impressions.
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Received on Sun Feb 11 2007 - 09:52:18 GMT
