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Re: [netcdf-java] 2 fix on GRIB1 reader

I guess I understand what's going on, Grib and Netcdf conventions declare their datas in a different order. 

So what do we choose ?
1 - leave it like it is and it's the user work to iterator in the inverse order if file_format = "GRIB-X" 2 - flip the datas directly in the reader to match netcdf order, ( for all grib projections, painful job ) 

Does someone know how it's handle in other readers ?

Johann Sorel
Geomatys


On 02/11/2012 12:25, johann sorel wrote:

Hello again,
I have a similar problem, this time on NOAA grib 2 data with LatLon projection. values are flipped along the latitude axis again (same as bug 2 I reported previously) 

this starts to make me think the problem might be in geotoolkit.
But usual netcdf files are displayed properly ...
I would like to have a confirmation, here is a partial dump obtained with class NCDumpW : What I would like to know is : the line marked #1 is for latitude +78 or -78 ? 

netcdf /..../somegrib.grib2 {
 dimensions:
   lon = 288;
   lat = 157;
   time = 61;
 variables:
   float lat(lat=157);
     :units = "degrees_north";
   float lon(lon=288);
     :units = "degrees_east";
   int time(time=61);
     :units = "Hour since 2012-10-30T06:00:00.000Z";
     :standard_name = "time";
float Wind_direction_from_which_blowing_degree_true_surface(time=61, lat=157, lon=288); :long_name = "Wind direction (from which blowing) (degree true) @ Ground or water surface"; 
     :units = "deg";
     :missing_value = NaNf; // float
     :abbreviation = "WDIR";
     :grid_mapping = "LatLon_Projection";
     :Grib_Variable_Id = "VAR_0-2-0_L1";
     :Grib2_Parameter = 0, 2, 0; // int
     :Grib2_Level_Type = 1; // int
     :Grib2_Generating_Process_Type = "Forecast";
....

 data:
lat =
{78.0, 77.0, 76.0, 75.0, 74.0, 73.0, 72.0, 71.0, ... , -74.0, -75.0, -76.0, -77.0, -78.0} 
lon =
{0.0, 1.25, 2.5, 3.75, 5.0, 6.25, 7.5, 8.75, 10.0, ... , 355.0, 356.25, 357.5, 358.75} 
time =
  {0, 3, 6, 9, 12, 15, 18, 21, 24, ... , 174, 177, 180}
Wind_direction_from_which_blowing_degree_true_surface =
  {
    {
      {56.78, 57.72, 61.87,... },    #1
      {56.78, 57.72, 61.87,... },
      {107.14, ... },
      ...
   }
}


thanks.

Johann Sorel




On 02/11/2012 02:18, John Caron wrote:

Hi johann :

Thanks very much, these fixes will be in target-4.3.14 later tonight.

John

On 10/31/2012 5:41 AM, johann sorel wrote:

Hello,

I work with martin desruisseaux on the geotoolkit.org project.

While trying to access datas from :
http://www.globalmarinenet.com/grib_downloads.php I found a few bugs in
the grib1 to netcdf metamodel.
Those are GRIB 1 files with LatLon projections.

grib/src/main/java/ucar/nc2/grib/grib1/Grib1Gds.java$LatLon

There was 2 bugs in this projection creation :

1 - a copy/paste error when calculating the latitude delta.

if (!Misc.closeEnough(deltaLat, calcDelta)) {
   log.debug("deltaLat != calcDeltaLat");
   deltaLat = calcDelta;  ----> was deltaLon = calcDelta;
}

2 - wrong value order when datas start by the lower values.

BEFORE
   if (deltaLat != GribNumbers.UNDEFINED) {
         deltaLat *= scale3; // undefined for thin grids
         if (la2 < la1) deltaLat *= -1.0;
   } else deltaLat = calcDelta;

AFTER
     if (deltaLat != GribNumbers.UNDEFINED) {
         deltaLat *= scale3; // undefined for thin grids
         if (la2 < la1) {
             //flip declaration order
             float latemp = la1;
             la1 = la2;
             la2 = latemp;
             calcDelta *= -1.0;

             //we must also consider the cell corner, since we flipped
the order
             //we should specify that the true value is at the
BOTTOM-LEFT corner
             //but we can't show this information so we make a one cell
displacement
             //to move the value on a TOP-LEFT corner.
             la1 -= calcDelta;
             la2 -= calcDelta;
         }
       } else {
           deltaLat = calcDelta;
       }



I attached a snapshot of the visualized datas before and after fix over
the indian ocean and the patched java class.


Johann Sorel
Geomatys





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