Java NetCDF Folks,
We have recently begun generating global 1/8 degree GRIB files
using NCEP's grid 174 definition
(http://www.nco.ncep.noaa.gov/pmb/docs/on388/tableb.html#GRID174).
Following that definition, we are setting the value of scanMode to 64, which
per GRIB2 Table 3.4 indicates "points scan in the +j direction", and so filling
the data arrays from south to north. However, the java-netCDF package (v4.5)
does not seem to handle the scanMode flag properly, and consequently our
grids appear flipped on display. Meanwhile, I understand that NCL does handle
the data mapping correctly, from which we conclude that java-netCDF is has a
problem with this grid definition.
As a work-around, we will set scanMode to 0 and write from north to south,
which will fix the problem for us. But, we'd also hope to have the tools work
correctly, too so that we can use the NCEP's grid as specified -- which
admittedly is strange in that it list the corner lons from north to south.
I've posted a sample GRIB2 file and resulting flipped image here:
ftp://aftp.fsl.noaa.gov/divisions/its/bob/grid174_scanmode_64_example.grb2
ftp://aftp.fsl.noaa.gov/divisions/its/bob/flipped_174grid.jpg
FWIW, here is the output from the (v4.3) Grib2Dump utility on that data file:
Header : GRIB2
Discipline : 0 Meteorological products
GRIB Edition : 2
GRIB length : 2808735
Originating Center : 59 The NOAA Forecast Systems
Laboratory
Originating Sub-Center : 0
Significance of Reference Time : 1 Start of forecast
Reference Time : 2015-01-08T00:00:00Z
Product Status : 0 Operational products
Product Type : 1 Forecast products
Number of data points : 4147200
Grid Name : 0 Latitude_Longitude
Grid Shape: 0 Earth spherical with radius =
6,367,470 m
Number of points along parallel: 2880
Number of points along meridian: 1440
Basic angle : 0
Subdivisions of basic angle: 0
Latitude of first grid point : 89.938
Longitude of first grid point : 0.062
Resolution & Component flags : 48
Winds : True
Latitude of last grid point : -89.938
Longitude of last grid point : 359.938
i direction increment : 0.125
j direction increment : 0.125
Grid Units : degrees
Scanning mode : 64
Product Definition : 0 Analysis/forecast at
horizontal level/layer at a point in time
Parameter Category : 0 Temperature
Parameter Name : 0 Temperature
Parameter Units : K
Generating Process Type : 2 Forecast
ForecastTime : 240
First Surface Type : 1 Ground or water surface
First Surface value : 0.0
Second Surface Type : 255 Missing
Second Surface value : 0.0
Thanks for any insights on this issue!
Bob Lipschutz
NOAA/ESRL/Global Systems Division
IT Services/Data Services Group
