[netCDF #YRC-237402]: netcdf4 dimensions output comment out

> Hi Russ,
> 
> Thanks for your explanation.
> 
> Recently JPSS program converted some hdf5 files into netcdf4 by removing
> some meta data
> information.
> 
> One example of output netcdf4:
> ftp://ftp.star.nesdis.noaa.gov/pub/corp/scsb/wchen/tmp/TATMS_npp_d20120911_t1359540_e1400256_b04529_c20120911202503549711_noaa_ops.nc
> 
> If viewed from hdfview, no those meta data, it's a netcdf4 file, right?  Or
> you think this still a
> hdf5 file?

It's a netCDF-4 file, as running the netCDF-4 ncdump utility shows:

  $ ncdump -k 
~/Downloads/TATMS_npp_d20120911_t1359540_e1400256_b04529_c20120911202503549711_noaa_ops.nc
  netCDF-4

Of course it's also an HDF5 file, because netCDF-4 files are just special kinds 
of 
HDF5 files with some added artifacts to support the netCDF-4 data model, which 
is 
different from the HDF5 data model.

> The input hdf5 used is:
> ftp://ftp.star.nesdis.noaa.gov/pub/corp/scsb/wchen/tmp/TATMS_npp_d20120911_t1359540_e1400256_b04529_c20120911202503549711_noaa_ops.h5

Yes, that's an HDF5 file that's not a netCDF-4 file, and the netCDF-4 file 
can't currently
read it, although the netCDF-4 library can read some HDF5 files that don't use 
the HDF5
features in this list:

  http://www.unidata.ucar.edu/netcdf/docs/faq.html#fv15

> That's the reason that lead me to think it's possible to skip meta data
> information in netcdf4 file.
> 
> All those red "A" on some original hdf5 fields are gone in output nc file.
> I guess they removed attributes.

h5dump should work on both files and make it clear what was removed.

--Russ

> address@hidden> wrote:
> 
> > Wanchun,
> >
> > > I used the simple C code: simple_xy_nc4_wr.c
> > >
> > http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-tutorial/simple_005fxy_005fnc4_005fwr_002ec.html#simple_005fxy_005fnc4_005fwr_002ec
> > >
> > >
> > >
> > http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-tutorial/simple_005fxy_005fnc4_005fwr_002ec.html#simple_005fxy_005fnc4_005fwr_002ec
> > >
> > > But generated nc file simple_xy_nc4.nc has x and y besides data when I
> > > viewed it in hdfview, but I only want data, not x and y.
> > >
> > > Can you tell me in which part of netcd4 source code do those dimensions
> > > like x and y get written out?   I want to disable them or comment them
> > > out so that only data is kept in file.  I searched netcdf4 src directory
> > > and can not pinpoint which is the correct code to change.
> >
> > What you're asking for is not possible, because of a mismatch between the
> > netCDF-4
> > data model and the HDF5 datamodel.  NetCDF-4 has "shared dimensions" such
> > as x
> > and y that can be shared among multiple variables, to indicate a common
> > grid.  HDF5,
> > used as the storage layer for netCDF-4, lacks the concept of shared
> > dimensions, but
> > provides a different mechanism. "dimension scales", on which netCDF-4
> > shared
> > dimensions are implemented.  An HDF5 dimension scale is a special type of
> > HDF5
> > dataset, and must always be used for every netCDF-4 dimension, because it
> > is the
> > only HDF5 mechanism available for modeling shared dimensions.
> >
> > If netCDF-4 had "anonymous dimensions" such as HDF5 dataspaces, which
> > cannot be
> > shared among variables, then you could define a variable that had its own
> > dataspace,
> > with unnamed and unsharable dimensions.  If you must store just the data
> > but not the
> > metadata about dimensions, you probably want to use HDF5 rather than
> > netCDF-4.
> >
> > For more on netCDF-4 dimensions versus HDF5 dimension scales, see these
> > blogs by John
> > Caron, explaining the issues:
> >
> >    http://www.unidata.ucar.edu/blogs/developer/en/entry/dimensions_scales
> >    http://www.unidata.ucar.edu/blogs/developer/en/entry/dimension_scale2
> >
> > http://www.unidata.ucar.edu/blogs/developer/en/entry/dimension_scales_part_3
> >
> > http://www.unidata.ucar.edu/blogs/developer/en/entry/netcdf4_shared_dimensions
> >
> > --Russ
> >
> > Russ Rew                                         UCAR Unidata Program
> > address@hidden                      http://www.unidata.ucar.edu
> >
> >
> >
> > Ticket Details
> > ===================
> > Ticket ID: YRC-237402
> > Department: Support netCDF
> > Priority: Normal
> > Status: Closed
> >
> >
> 
> 
Russ Rew                                         UCAR Unidata Program
address@hidden                      http://www.unidata.ucar.edu



Ticket Details
===================
Ticket ID: YRC-237402
Department: Support netCDF
Priority: Normal
Status: Closed