Hi Bryan:
1. The classes you are working with in NetCDF-Java 4.0 are no longer
supported. Please look at ucar.nc2.grib.* classes in NetCDF-Java 4.3.
2. NetCDF-Java does not support writing GRIB files, only reading them.
So we support GRIB -> netcdf/CF, but not the other way around. In
principle, if you understand the CF conventions, you can write the
correct GRIB GDS from them.
3. The convention that projection x and y values must be positive is
not used. Thats more of a GIS thing and is rather artificial; one
typically has to add "false_northing" and "false_easting" to make it true.
4. The "origin of the projection" may also be different than what you
are used to. It is essentially the place where x=0, y=0 in the
projection defined by the LambertConformal class; LA1 and LO1 is the
position of one of the corners of the grid. Theres no documentation
other than the code for GRIB, but you might find this helpful:
http://www.unidata.ucar.edu/software/netcdf-java/reference/StandardCoordinateTransforms.html
John
On 6/25/2012 12:14 PM, Rockwood, Bryan wrote:
Greetings,
As part of the process of creating a GRIB1 file from subgridded netCDF data
generated by the NetCDF-Java 4.0 API (long story), I had a difficult time
figuring out how the X and Y values from the netCDF were calculated. Since
there were negative X and Y values, it appeared that the projection used to
calculate X and Y were not grid based. So, I obtained the Java netCDF source
and proceeded to debug through the code. After some exploring, I found a class
called GridHorizCoordSys in the package ucar.nc2.iosp.grid. In said class is a
private method called makeLC and the first thing it does is create a projection
for the GRIB record. Now here's where I'm confused. This is the code in
question:
proj = new LambertConformal(
gds.getDouble(GridDefRecord.LATIN1),
gds.getDouble(GridDefRecord.LOV),
gds.getDouble(GridDefRecord.LATIN1),
gds.getDouble(GridDefRecord.LATIN2));
Looking at the javadoc for the LambertConformal constructor, the first two
parameters should be the latitude and longitude origin of the coordinate
system. In my experience when dealing with Lambert Conformal grids, this is
typically GridDefRecord.LA1 and GridDefRecord.LO1, respectively, from the GRIB
GDS. The code above is using GridDefRecord.LATIN1 and GridDefRecord.LOV
instead. Once I found out how the projection was being created, it was then
possible to get the LA1 and LO1 from the X and Y within the netCDF to create a
valid GDS for the GRIB data I'm generating. My questions are as follows:
Why use the LATIN1 and LOV to set the lat and lon origin of the projection
instead of LA1 and LO1?
Is this documented someplace? I'm curious how someone such as myself would
know to create a projection like this without debugging through the code.
Bryan Rockwood
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