Dear Phil et al.,
I too know very little about this, but as far as I know:
1) observational datasets used to evaluate model simulations also don't
supply this information.
2) As Jonathan, says models are idealized. They also sometimes rely on
an idealized calendar (e.g., assuming each year comprises twelve 30-day
months). We have allowed for this in the CF convention. Perhaps, we
should also allow for this in specifying the "datum" by simply
indicating an idealized earth shape?
3) As we move to higher and higher resolution, the issue will certainly
become of more concern, so it's good we think about this now.
I second Jonathan's plea for some experts to weigh in.
cheers,
Karl
Jonathan Gregory wrote:
> Dear Phil
>
>> non-geodesist interpretation of a geodetic datum is that it defines the
>> mapping of a Coordinate System (such as latitude-longitude) onto a
>> particular figure of the earth within a given reference frame, thus
>> providing a Coordinate Reference System.
>
> That sounds similar to my interpretation (that it defines an ellipsoid).
>
>> Although a spherical earth approximation may be appropriate for many
>> climate applications, I guess there may be plenty of situations where
>> meteorological datasets are actually referenced to a non-spherical
>> earth, e.g. met observations referenced against the popular WGS 1984
>> datum/ellipsoid. In such cases I would have thought that the geodetic
>> datum information ought indeed to be captured.
>
> Yes, if the dataset had one, but I'm sure that most lat-lon meteorological
> datasets (the ones I've come across, anyway) do not specify their datum.
> They may have been gridded from obs, or they may provide obs on whatever
> lat-lon system the country concerned uses. That implies a kind of mixture, I
> suppose, which is OK because the differences don't matter for the purposes for
> which the dataset is intended. But I really don't know about this. I'm sure
> someone on the conventions committee must be an expert! For climate model
> output the datum is genuinely undefined since the world is idealised.
>
>> As regards the content and structure of a potential 'geodetic_datum'
>> grid mapping attribute, would a solution be to adopt the well-known text
>> format devised by the Petrotechnical Open Standards Consortium (POSC)
>> and since adopted by the Open Geospatial Consortium (OGC). In Backus-
>> Naur Form this looks thus:
>>
>> <datum> ::=
>> DATUM [ "<name>", <spheroid>
>> {, <shift-x>, <shift-y>, <shift-z>, <rot-x>, <rot-y>, <scale-
>> adjust> }
>> ]
>>
>> <spheroid> ::=
>> SPHEROID ["<name>", <semi-major-axis>, <inverse-flattening>]
>>
>> And so the WGS 1984 datum example might then look something like:
>>
>> DATUM [ "World Geodetic System 1984" SPHEROID ["WGS 1984", 6378137.00
>> 298.257223563 ] ]
>
> This looks promising. To make it more self-describing and easier to use I
> suggest we might put those elements of the definition in separate attributes.
> Is it really that simple though? I would have expected more numbers: perhaps
> that's what the <shift>s and <rot>s are.
>
> My ignorance of this is all too obvious. To proceed we need expertise and
> use-cases.
>
> Cheers
>
> Jonathan
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Received on Tue Feb 20 2007 - 10:20:08 GMT
