Hi Jonathan,
I suspect that the term "datum" means different things to different
people and communities. The documentation that accompanies the EPSG
geodetic parameters database does a pretty good job of describing the
nature and purpose of classical geodetic datums. My highly paraphrased,
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.
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.
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 potential solution avoids the use of external tables (thus
fulfilling the CF self-describing remit), yet still manages to adhere to
a recognised, de facto standard, one that has been widely adopted.
Just a thought!
Regards,
Phil
> 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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