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?

--
--------------------------------------------------------------
Dr Jon Blower              Tel: +44 118 378 5213 (direct line)
Technical Director         Tel: +44 118 378 8741 (ESSC)
Reading e-Science Centre   Fax: +44 118 378 6413
ESSC                       Email: address@hidden
University of Reading
3 Earley Gate
Reading RG6 6AL, UK
--------------------------------------------------------------

===============================================================================
To unsubscribe netcdf-java, visit:
http://www.unidata.ucar.edu/mailing-list-delete-form.html
===============================================================================