Re: [netcdf-java] Help reading grib2 file. (fwd)

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Robb Kambic                                Unidata Program Center
Software Engineer III                      Univ. Corp for Atmospheric Research
address@hidden             WWW: http://www.unidata.ucar.edu/
===============================================================================

---------- Forwarded message ----------
Date: Fri, 2 Oct 2009 13:14:41 -0600 (MDT)
From: Robb Kambic <address@hidden>
To: Esteban Wagner <address@hidden>
Subject: Re: [netcdf-java] Help reading grib2 file.

On Tue, 29 Sep 2009, Esteban Wagner wrote:

> Hi, 
> I have a grb2 file containing global Primary_wave_direction forecasts. I 
> would like to read the
> values it contains.
>
> By the moment I was able to read the file and obtain a GridDataset using the 
> following code:
> ------------
> (NetCDF:Version 4.0.16; Grib:Version 6.0.16)
>
> final File file = new File("src/test/resources/multi_1.glo_30m.DIRPW.grb2");
> final GridDataset gridDataSet = GridDataset.open(file.getAbsolutePath());
> ----------
> the file can be downloaded from: 
> ftp://polar.ncep.noaa.gov/pub/waves/develop/multi_1.latest_run/multi_1.glo_30m.DIRPW.grb2
> ----------
> Now I would like if someone can tell me which are the methods from 
> gridDataSet I should use in order
> to obtain the different forecast values of the Primary_wave_direction for a 
> given grid point.
>
> Thanks,
>
> Regards.
>
> Esteban.
Esteban,

I'll reference the JavaDocs first:

http://www.unidata.ucar.edu/software/netcdf-java/v4.1/javadoc/ucar/nc2/dt/GridDataset.html
http://www.unidata.ucar.edu/software/netcdf-java/v4.1/javadoc/ucar/nc2/dt/GridDatatype.html
http://www.unidata.ucar.edu/software/netcdf-java/v4.1/javadoc/ucar/nc2/dt/GridCoordSystem.html

I'm assuming by grid point you mean Lat/Lon
One needs to be at the GridCoordSystem level to be able to convert the Lat/Lon 
point into indices. Then  use the indices information in the GridDatatype 
readDatSlice routine to get the value. Here's a short sample program.

public class GetDataFromLatLon {
  public static void main(String[] args) {

       try {

        final File file = new File("C:/data/grib/multi_1.glo_30m.DIRPW.grb2");
        final GridDataset gridDataSet = 
GridDataset.open(file.getAbsolutePath());
        GridDatatype pwd = gridDataSet.findGridDatatype( 
"Primary_wave_direction");
        System.out.println( pwd.getInfo());

        GridCoordSystem pwdGcs = pwd.getCoordinateSystem();
        int[] result = null;
        // Get index value for Lat 30.0 and Lon 179
        int[] idx = pwdGcs.findXYindexFromLatLon( 30.0, 179.0, result );
        //int[] idx = pwdGcs.findXYindexFromLatLon( 90.0, 0.0, result );
        // Extract data value for time 0, no Vert index, Lat index, Lon index
        Array data = pwd.readDataSlice( 0, -1, idx[1], idx[0]);
        // Another option: read all data values for timeIndex = 0
        //Array data = pwd.readVolumeData( 0 );
        IndexIterator iter = data.getIndexIterator();
        while(iter.hasNext()) {
                float val = iter.getFloatNext();
                System.out.println( "Primary_wave_direction = "+ val);
        }

        System.out.println( "Success");
       } catch (Exception exc) {
              exc.printStackTrace();
       }
    }
}
===============================================================================
Robb Kambic                                Unidata Program Center
Software Engineer III                      Univ. Corp for Atmospheric Research
address@hidden             WWW: http://www.unidata.ucar.edu/
===============================================================================