[netcdf-java] Grib1 Gaussian Grid Relative Humidity Interpolation Issue

```Hi all,

we’re implementing some service that reads ECMWF 0.125° Grib files in gaussian
grid. The NetCDF-Library automatically interpolates the gaussian grid to a
regular lat/lon-grid. For the relative humidity we found some strange values in
output after that interpolation.

Using wgrib I’ve extracted some isobaric relative humidity record to a single
grib file called „dpd_test.grib“ (which i could send as well if needed).
According to wgrib, it is an gaussian grid and min and max values are 0% and
100% - as I would expect them for a relative humidity (see attached output).

Next I’ve written a small program which extracts that relative humidity
parameter via netcdf-java-lib and step thru all values to find minimum and
maximum in it. Source is also attached - just in case I did something wrong
there.

The output of that states the following...
2013-11-14 13:45:15,378 INFO : App -
dpd_test.grib/Relative_humidity_isobaric/850.0hPa: min=-4.462653160095215
max=108.53846740722656
…indicating the minimum relative humidity is around -4.46% and the maximum at
108.54%.

Browsing your code on Github I think you’re using cubic interpolation to
convert gaussian to regular grid, which might cause this problem.
https://github.com/Unidata/thredds/blob/6b1052455e597797f3a5980165a23292172920ce/grib/src/main/java/ucar/nc2/grib/QuasiRegular.java
Line 219 - 221:
outpt[oIdx] = (float) (a * inpt[iIdx + low] + b * inpt[iIdx + hi]
+ ((a * a * a - a) * y2d[low]
+ (b * b * b - b) * y2d[hi]) / 6.0);
Is the problem in the parametrization of this formula?

Is it possible to use Linear Interpolation here?

Kind Regards,
Jochen

— wgrib output ---------------
Losty-MacBook:Desktop jkaehler\$ ~/wgrib/wgrib -V dpd_test.grib
rec 1:0:date 2013111300 R kpds5=157 kpds6=100 kpds7=850 levels=(3,82) grid=255
850 mb 6hr fcst:
R=Relative humidity [%]
timerange 0 P1 6 P2 0 TimeU 1  nx -1 ny 800 GDS grid 4 num_in_ave 0 missing 0
center 98 subcenter 0 process 143 Table 128 scan: WE:NS winds(N/S)
thinned gaussian: lat  89.828000 to -89.828000
long 0.000000 to 359.900000, 843490 grid pts   (-1 x 800) scan 0 mode
0 bdsgrid 1
18   25   32   40   45   50   60   60   72   72   75   81   90   96  100
108  120  120  125  128  144  144  150  160  160  180  180  192  192  200
200  216  216  225  240  240  240  250  250  256  270  288  288  288  300
300  320  320  320  324  360  360  360  360  360  360  375  375  384  400
400  400  405  432  432  432  432  450  450  450  480  480  480  480  480
486  500  500  512  512  540  540  540  540  540  576  576  576  576  576
576  600  600  600  600  640  640  640  640  640  640  640  648  675  675
675  675  675  720  720  720  720  720  720  720  729  729  750  750  750
750  768  768  768  800  800  800  800  800  800  810  864  864  864  864
864  864  864  864  864  864  900  900  900  900  900  900  900  960  960
960  960  960  960  960  960  960  960  960  960  972  972 1000 1000 1000
1000 1000 1000 1024 1024 1024 1024 1024 1080 1080 1080 1080 1080 1080 1080
1080 1080 1080 1080 1125 1125 1125 1125 1125 1125 1125 1125 1125 1152 1152
1152 1152 1152 1152 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200
1215 1215 1215 1215 1280 1280 1280 1280 1280 1280 1280 1280 1280 1280 1280
1280 1280 1280 1280 1280 1296 1296 1296 1296 1350 1350 1350 1350 1350 1350
1350 1350 1350 1350 1350 1350 1350 1350 1350 1440 1440 1440 1440 1440 1440
1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440
1440 1440 1440 1440 1440 1440 1440 1458 1458 1458 1458 1458 1458 1458 1500
1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500
1500 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536
1536 1536 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600 1600 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536
1536 1536 1536 1536 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500
1500 1500 1500 1500 1500 1500 1458 1458 1458 1458 1458 1458 1458 1440 1440
1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440
1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1350 1350 1350 1350
1350 1350 1350 1350 1350 1350 1350 1350 1350 1350 1350 1296 1296 1296 1296
1280 1280 1280 1280 1280 1280 1280 1280 1280 1280 1280 1280 1280 1280 1280
1280 1215 1215 1215 1215 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200
1200 1152 1152 1152 1152 1152 1152 1125 1125 1125 1125 1125 1125 1125 1125
1125 1080 1080 1080 1080 1080 1080 1080 1080 1080 1080 1080 1024 1024 1024
1024 1024 1000 1000 1000 1000 1000 1000  972  972  960  960  960  960  960
960  960  960  960  960  960  960  900  900  900  900  900  900  900  864
864  864  864  864  864  864  864  864  864  810  800  800  800  800  800
800  768  768  768  750  750  750  750  729  729  720  720  720  720  720
720  720  675  675  675  675  675  648  640  640  640  640  640  640  640
600  600  600  600  576  576  576  576  576  576  540  540  540  540  540
512  512  500  500  486  480  480  480  480  480  450  450  450  432  432
432  432  405  400  400  400  384  375  375  360  360  360  360  360  360
324  320  320  320  300  300  288  288  288  270  256  250  250  240  240
240  225  216  216  200  200  192  192  180  180  160  160  150  144  144
128  125  120  120  108  100   96   90   81   75   72   72   60   60   50
45   40   32   25   18
min/max data 0 100  num bits 8  BDS_Ref 0  DecScale 0 BinScale -1
——

— Java-Source ——
GridDataset dataset = GridDataset.open(gribFile.getAbsolutePath());

try {
GridDatatype grid =
dataset.findGridDatatype("Relative_humidity_isobaric");
if(grid != null) {
CoordinateAxis1D isbl =
grid.getCoordinateSystem().getVerticalAxis();
double[] isblValues = isbl.getCoordValues();
for (int isblIdx = 0; isblIdx < isblValues.length; isblIdx++) {
double[] data = (double[]) grid.readDataSlice(0,
isblIdx, -1, -1).get1DJavaArray(double.class);
double min = Double.MAX_VALUE;
double max = Double.MIN_VALUE;
for (double d : data) {
min = Math.min(min, d);
max = Math.max(max, d);
}
LOG.info("{}/{}/{}{}: min={} max={}",
gribFile.getName(), grid.getFullName(), isblValues[isblIdx],
isbl.getUnitsString(), min, max);
}
}
} finally {
if (dataset != null) {
dataset.close();
}
}

--------```

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