Re: [netcdf-java] Reading GRIB2 levels data

• To: "netcdf-java@xxxxxxxxxxxxxxxx" <netcdf-java@xxxxxxxxxxxxxxxx>
• Subject: Re: [netcdf-java] Reading GRIB2 levels data
• From: "Dahn, Nikolas" <nikolas.dahn@xxxxxxxxxxxxxxxxxxxxxx>
• Date: Mon, 5 Dec 2016 16:50:10 +0000

I found a fix and a way to apply it as a temporary hack. Basically what happens 
is the following:
- In Grib2CollectionBuilder.Grib2Rectilyser.make unique coordinate systems are 
built from the various PDS sections
- The PDS' data sections are collected into VariableBags
- To determine which coordinates are used the vertical height unit is examined, 
among others
- This unit is (usually) provided by the Grib2Customizer (an implementation of 
GribTables)
- The current implementation lacks a switch-case for code 150 (generalized 
vertical height) and the default mapping defines the unit as 'null'
- From this it is determined that no vertical coordinate is present
- Thus the coordinate is missing and the builder builds the coordinate system 
without the height coordinate
- In the end most of the data is missing because the later data sections 
overwrite the earlier ones, as one of their distinguishing dimensions is missing

The fix is quite simple: all I did was adding case statements for code 150 in 
Grib2Customizer.getVertUnit(int) and Grib2Customizer.getLevelNameShort(int). A 
patch is attached below.

For those who stumble over this and don't have access to a patched version, you 
can derive a class from Grib2Customizer and override the two offending methods. 
Then set a private variable of Grib2Customizer via reflection like shown below. 
I recommend doing this in a static block.

    Field f = Grib2Customizer.class.getDeclaredField("wmoStandardTable");
    f.setAccessible(true);
    f.set(null, MyGrib2Customizer.factory(0, -1, -1, -1, -1));

Cheers,
~


Patch:
====
--- ucar/nc2/grib/grib2/table/Grib2Customizer.java.orig    2016-12-05 
17:44:09.663099300 +0100
+++ ucar/nc2/grib/grib2/table/Grib2Customizer.java.patched    2016-12-05 
17:45:16.147099300 +0100
@@ -408,6 +408,9 @@
       case 119:
         return new GribLevelType(code, "Pa", null, false); // ??

+      case 150:
+        return new GribLevelType(code, "numeric", null, true);
+
       case 160:
         return new GribLevelType(code, "m", "sea level", false);

@@ -505,6 +508,8 @@
         return "hybrid_pressure";
       case 120:
         return "pressure_thickness";
+      case 150:
+        return "generalized_vertical_height";
       case 160:
         return "depth_below_sea";
       case GribNumbers.UNDEFINED:

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  • Re: [netcdf-java] Reading GRIB2 levels data
    • From: John Caron