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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 > > _______________________________________________ > netcdf-java mailing list > netcdf-java@xxxxxxxxxxxxxxxx > For list information or to unsubscribe, visit: > http://www.unidata.ucar.edu/mailing_lists/
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