Hi Ben:
I see you guys are making good progress on this.
The question I never answered for myself is what is the relationship
between
ucar.unidata.geoloc.projection.RotatedPole
which was contributed for CF:
/**
* Rotated-pole longitude-latitude grid.
*
* This is probably the same as rotated lat lon, using matrix to do rotation.
* Follows CF convention with "north_pole lat/lon", whereas RotatedLatLon
uses south pole.
*
* @author Robert Schmunk
* @author jcaron
*/
and
ucar.unidata.geoloc.projection.RotatedLatLon
which was contributed for GRIB data:
/**
* Grib 1 projection 10 and Grib 2 projection 1.
*
* The Rotated Latitude Longitude projection algorithms that are coded
* here were given by Tor Christian Bekkvik <torc@xxxxxx>. The rotated
* lat/lon projection coordinates are defined in the grid file that
* need to be converted back to unrotated lat/lon projection coordinates
* before they can be displayed. The X/Y axis only makes sense in the
rotated
* projection.
*
* @author Tor Christian Bekkvik <torc@xxxxxx>
* @since Nov 11, 2008
*/
Some of your questions answers are probably lurking in the difference
between the two.
John
PS: I presume youve given us some test GRIB files using template 32769
to add to our unit tests?
On Thu, Apr 14, 2016 at 4:15 PM, Ben Caradoc-Davies <ben@xxxxxxxxxxxx>
wrote:
> FYI I have submitted a pull request (and Christian found some test
> failures in the Unidata suite):
> https://github.com/Unidata/thredds/pull/531
>
> Kind regards,
> Ben.
>
>
> On 14/03/16 14:23, Ben Caradoc-Davies wrote:
>
>> John,
>>
>> I have partly implemented NetCDF-Java support for rotated
>> latitude/longitude GRIB2 files with the unusual NCEP GDS template 32769:
>> http://www.nco.ncep.noaa.gov/pmb/docs/grib2/grib2_temp3-32769.shtml
>>
>> Although this is an unusual GDS, my investigations indicate that it is
>> the same projection as GDS template 1 but defined by different GDS
>> parameters. I am thus keen to reuse the existing implementation
>> Grib2Gds.RotatedLatLon to reduce code duplication.
>>
>> During implementation, I encountered some behaviour of GribIosp and
>> Grib2Gds.RotatedLatLon that I do not understand:
>>
>> (1) Grib2Gds.RotatedLatLon appears to make no transformation to the
>> rotated coordinate system. The generated lat/lon will extend from
>> la1/lo1 to la2/lo2 (the unrotated corners). What happened to the new
>> rotated origin? I expected the rotated grid to centred on (0,0) if the
>> southern pole parameters set the origin as the centre of the grid (not
>> mandatory but a popular choice):
>>
>> https://github.com/Unidata/thredds/blob/master/grib/src/main/java/ucar/nc2/grib/grib2/Grib2Gds.java#L535
>>
>>
>> (2) The coordinate variables created in GribIosp for rotated
>> longitude/latitude GRIB2 grids do not appear to conform to the CF
>> Conventions as they lack rlat/rlon and standards_name
>> grid_latitude/grid_longitude and do not contain multidimensional lat/lon
>> as required. Instead they appear to contain a simple lat/lon as for an
>> unrotated grid:
>>
>> http://cfconventions.org/cf-conventions/cf-conventions.html#grid-mappings-and-projections
>>
>>
>> (3) Rather than using GdsHorizCoordSys.isLatLon as for GRIB1, for GRIB2
>> files GribIosp uses GDS template range checks to decide if a grid is
>> lat/lon. This seems to lack extensibility. What is the reason for this
>> implementation choice, and can I change it?
>>
>> https://github.com/Unidata/thredds/blob/master/grib/src/main/java/ucar/nc2/grib/collection/GribIosp.java#L240
>>
>>
>> https://github.com/Unidata/thredds/blob/master/grib/src/main/java/ucar/nc2/grib/grib2/Grib2Utils.java#L116
>>
>>
>> Is anyone using NetCDF-Java support for GDS template 1 GRIB2 files? Does
>> anyone have any sample files for GDS template 1 and known good NetCDF CF
>> Conventions equivalents?
>>
>> I suspect the NetCDF-Java implementation of GDS template 1 may be
>> incorrect; if I am right, I intend to fix it as a side-effect of
>> implementing support for GDS 32769. If I am wrong, please stop me now!
>>
>> Kind regards,
>>
>>
> --
> Ben Caradoc-Davies <ben@xxxxxxxxxxxx>
> Director
> Transient Software Limited <http://transient.nz/>
> New Zealand
>
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