On 4/24/2012 4:16 AM, Hedley, Mark wrote:
There are particular issue here with format interoperability and conversion
with respect to phenomenon. In GRIB2, for example, there are codes which
identify vector components:
Wind direction (from which blowing) degree true
Wind speed ms
u-component of wind ms
v-component of wind
but no code for eastward_...
The current situation poses significant challenges for GRIB - CF
interoperability.
On 4/24/2012 10:42 AM, Hedley, Mark wrote:
> Can similar tests be used with Grib conversion? I assume that Grib must
provide sufficient metadata to indicate whether the grid is lon-lat.
Yes and your assumption is correct, there is sufficient information to deliver
interoperability, but implementing this is adding complexity which I feel is
not necessary and may cause future issues maintaining the special cases. One
of the reasons I raise GRIB is that it treats vector component as one
phenomenon, as I have described, not two as is done in CF currently.
A few comments on my favorite topic, GRIB:
AFAIU, like CF, GRIB does not associate, say, the u and v components of
wind together, or identify them as vectors. You "just have to know" what
the naming conventions are to make the association.
More generally, it seems that the crux of the issue is that one wants to
know if the vector components of, say, wind, are reletive to the grid or
to lat/lon. GRIB does encode this info in the GDS, the equivalent of
the CF projection. Again you just have to know if that information is
actually pertinent to the GRIB record you are looking at. If you know
this, one could use it in the correct standard CF name.
In terms of mapping between GRIB and CF standard_names, these is
unfortunately no such up-to-date mapping, AFAIK. It would be very useful
and quite challenging to do so. One reason its hard is that a GRIB
"variable" must be discovered through hueristics that involve the
parameter name, the level type, and a whole bunch of other things that
you "just have to know". Some details for the foolhardy are at a URL you
just have to know, but since I know it, here it is:
http://www.unidata.ucar.edu/blogs/developer/en/entry/dataset_schemas_are_lost_in
John
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