Arrays with null entries?

[Simon Wood]

Hi Gaby,

> I am new to NetCDF format, so please bear with me...
> When I currently instantiate an array it
> zeros out all entries, is there a way to avoid that? I don't see
> anything in the javadocs that allows me to do this.
> When I write out the array to a file, what I'd like to see in the output
> file when I invoke ncdump is entries like _, _, _, ...  where there is
> no data.

I guess this must be something to do with fill mode and the value of 
_FillValue.  How are you creating the file?  if 
NetcdfFileWriteable.createNew() or openExisting() what fill mode are you 
using?  With fill mode enabled the array should get pre-filled with 
whatever '_FillValue' is set to for the variable in question.  If you 
don't want this behavior then set fill mode to false.

according to ncdump man page:
   ncdump uses ‘_’ to represent data values that are equal to the
   _FillValue’ attribute for a variable, intended  to  represent  data
   that  has  not  yet been written.  If a variable has no ‘_FillValue’
   attribute, the default fill value for the variable type is used if the
   variable is not of byte type.

I would infer from that that if you do want '_' used to represent 
missing data then you need fill mode enabled.  Further the _FillValue 
used will have to match ncdump's defaults (or be stated explicitly in 
your netCDF file).

I'm not sure what NJ22 uses as default _FillValues (NaN for floats?, not 
sure about ints), but it is possible that NJ22's defaults do not match 
ncdump's expected values (being the C defaults defined in netcdf.h, I 
assume).

Better practice might be to explicitly define a fill value outside the 
valid range for your data and add the _FillValue attribute to the 
variable.  That way ncdump should always recognise it whatever value you 
use.

Hope this helps

Simon

-- 
Simon Wood
Scientific Programmer (Remote Sensing)
National Institute of Water and Atmospheric Research, Wellington, NZ	
simon.wood at niwa.co.nz
http://www.niwa.co.nz

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