Re: [netcdf-java] GRIB-2-netCDF Projection Question


Thanks for the quick response.

1.       Sadly, I'm just a recipient of the netCDF files created by the v4.0 
API and the upstream source of this data is not planning on upgrading anytime 
soon.  That said, I expected you guys might say that so I did verified that the 
code works this way in 4.3 too.

2.       Understood.  I've already finished the code that does the netCDF -> 
GRIB conversion so this was more of a "why does it work that way?" type of 

3.       Gotcha.  I wasn't even thinking of it from a GIS standpoint.  After 
spending time  writing code that handles GRIB data, I'm used to the first point 
in a GRIB record being 0,0 and going from there.  The only reason I brought it 
up was that it was the first indication that the projection being used to 
generate the x and y is not using LA1 and LO1 as the originating point.

4.       I'm guessing this is where I'm getting hung up.  All this time, I've 
been working with  the assumption that LA1 and LO1 from the GDS was origin of 
the projection.  How was it decided that LATIN1 and LOV were going to be used 
for the origin instead?  I've talked to a two other software devs who work with 
GRIB (full disclosure, one is a meteorologist, the other is not), and they 
haven't seen it done this way either.

I appreciate the input here.  To be fair, now that I understand how it works, 
my conversion code spits out great GRIB formatted messages and, as I mentioned 
above, this was primarily a follow up to see why it is done this way and 
determine if there's a problem.


From: netcdf-java-bounces@xxxxxxxxxxxxxxxx 
[mailto:netcdf-java-bounces@xxxxxxxxxxxxxxxx] On Behalf Of John Caron
Sent: Monday, June 25, 2012 1:30 PM
To: netcdf-java@xxxxxxxxxxxxxxxx
Subject: Re: [netcdf-java] GRIB-2-netCDF Projection Question

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:


On 6/25/2012 12:14 PM, Rockwood, Bryan wrote:


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 

 proj = new LambertConformal(



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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