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[THREDDS #TKC-172853]: Missing values and TDS

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  • Subject: [THREDDS #TKC-172853]: Missing values and TDS
  • Date: Tue, 18 Oct 2011 12:30:15 -0600
> Ethan
> 
> I am using opendap access to the NOMADS server. I open the files in NCL
> and then read them. I also tested open the files using dncdump
> 
> dncdump -h
> http://nomads.ncdc.noaa.gov/thredds/dodsC/cfsrmon/197901/ocnh06.gdas.197901.grb2
> 
> and see NaN as the missing value there. e.g.
> 
> ......
> float Vertical_velocity_geometric(time, depth_below_sea, lat, lon) ;
> Vertical_velocity_geometric:long_name =
> "Vertical_velocity_geometric @ depth_below_sea" ;
> Vertical_velocity_geometric:units = "m s-1" ;
> Vertical_velocity_geometric:missing_value = NaNf ;
> .......
> 
> We downloaded the grib file directly and looked at it. It does not have
> the NaN for any of the values as there is a mask applied when the grib
> is converted to netcdf (in this case, a land mask).
> 
> So, it is how the grib files are being converted to netCDF, as you suggest.
> 
> Let me know if I can answer any other questions.
> 
> Cathy

Hi Cathy:

The rib file does not store the missing values, so each library chooses that 
representation. The netcdf-java library uses NaNs to indicate missing values. 
This currently cannot be changed. What problem are you having with portability 
of NaNs?

John

> 
> On 9/28/11 10:31 AM, Unidata THREDDS Support wrote:
> > Hi Cathy,
> >
> > How are you getting the data? Are you using the NetCDF Subset Service 
> > (NCSS) access method to request the dataset?
> >
> > There is no way to specify that you would like non-NaN missing values when 
> > you request a netCDF file from the TDS. I suspect it isn't a simple matter 
> > in this case as it has to do with how the GRIB files are being converted 
> > into netCDF.
> >
> > However, I'm going to ask John Caron to give a more definitive answer. 
> > Unfortunately he won't be back from vacation for about two weeks.
> >
> > Ethan
> >
> > Cathy Smith wrote:
> >> I am reading CFSR data from NOMADS and missing values are being returned
> >> as NaN (for example, for  ocean variables over land) from the TDS. This
> >> causes problems for me in NCL and likely would in other languages.  Can
> >> missing values be returned as something more portable such as 9.9692e+36
> >> as is the default in C?
> >> Thanks,
> >> Cathy Smith
> >
> > Ticket Details
> > ===================
> > Ticket ID: TKC-172853
> > Department: Support THREDDS
> > Priority: High
> > Status: Open
> >
> 
> --
> ----------------------------------------------
> NOAA/ESRL PSD and CIRES CDC
> 303-497-6263
> http://www.esrl.noaa.gov/psd/people/cathy.smith/
> 
> Emails about data/webpages may get quicker responses from emailing
> address@hidden
> 
> 


Ticket Details
===================
Ticket ID: TKC-172853
Department: Support THREDDS
Priority: Critical
Status: Open