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Re: Problems reading NetCDF data - get NaNs

Hi Don,

Thanks for your quick reply.  I switched to 2.2.17 and the problem is
still there.  The values of temperature from ncdump include (file is
large!):

32767, 32767, 32767, 32767, 32767, 32767, 32767, 32767, 32767, -20974,
   -20974, -20979, -20985, 32767, 32767, 32767, 32767, -21054, -21072,
   -21083, -21093, -21101, -21108, -21112, -21115, 32767, 32767, 32767,

32767 represents missing data (metadata has :_FillValue = 32767;) and,
for example, -21072 represents (-21072 * 0.001 + 20=) -1.072 degrees
C, which is within the valid range.

According to the file metadata, the convention is COARDS.  Also, as I
said in my original post, ncBrowse displays the data without any
problems.

Thanks again,
Jon

On 19/10/06, Don Murray <address@hidden> wrote:

Hi Jon-

I suspect the problem is with the valid_max and valid_min
attributes.  The library will return NaN if the
values are outside the range of the min/max.  If the
type of the attribute is the same as the variable, they
are compared before scale and offset.  If they are different
they are compared after scaling and offset.  I recently
found a bug with this that was fixed in the 2.2.17 pre-release,
but I'm not sure if that is why you are seeing what you
are seeing.

What are the values for temperature in ncdump?

Don
*************************************************************
Don Murray                               UCAR Unidata Program
address@hidden                        P.O. Box 3000
(303) 497-8628                              Boulder, CO 80307
http://www.unidata.ucar.edu/staff/donm
*************************************************************



Jon Blower wrote:
> Dear all,
>
> I'm having some problems reading data from a particular NetCDF file.
> The code i'm using is:
>
>  NetcdfDataset nc =
> NetcdfDataset.openDataset("C:\\data\\OA_20060830.nc", true, null);
>  GridDataset gd = new GridDataset(nc);
>  GeoGrid gg = gd.findGridByName("temperature");
>  Array arr = gg.readYXData(0, 0);
>  IndexIterator it = arr.getIndexIteratorFast();
>  while (it.hasNext()) {
>      double val = it.getDoubleNext();
>      System.out.println("" + val);
>  }
>  nc.close();
>
> I just get a load of NaNs, even though I know that there are valid
> data in the file (ncBrowse displays the data perfectly).  The only
> unusual thing about the data is that the data are stored as short
> integers, with an offset and scale factor:
>
>     short temperature(time, depth, latitude, longitude);
>        :long_name = "Temperature";
>        :missing_value = 32767; // short
>        :_FillValue = 32767; // short
>        :units = "degree_Celsius";
>        :valid_min = -3; // short
>        :valid_max = 40; // short
>        :add_offset = 20.0; // double
>        :scale_factor = 0.0010; // double
>        :comment = "Temperature estimate (by objective analysis)";
>
> I understood that the NetcdfDataset class automatically dealt with
> offsets and scale factors  in "enhanced" mode, which is what I am
> using (or trying to use).  Hence I was expecting to "see" the data as
> an array of doubles.  The same happens if I use getFloatNext() or
> getShortNext().
>
> Can anyone see what I'm doing wrong?


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