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Re: netcdf and hdf

> Organization: BB&N
> Keywords: 199405041917.AA24090

Hi Ken,

> Do you know whether there is any relationship or comparison to be made
> between Unidata netCDF format and NCSA HDF format?  E.g. do they solve
> different problems or are they independent attacks on similar problems?
> are they complementary and worth synthesizing? is one a subset of the
> other?
> 
> I came to wonder about this because NCSA (the makers of Mosaic) makes
> a conferencing application called collage, which uses HDF for scientific
> data representation... but I don't know much more than that.


First, here's the official answer from the netCDF FAQ at
http://www.unidata.ucar.edu/packages/netcdf/faq.html

 7. What is the connection between netCDF and HDF? 

    The National Center for Supercomputing Applications (NCSA) developed the
    HDF software and makes it freely available. HDF is an extensible data
    format for self-describing files that was developed independently of
    netCDF. Applications and utilities based on HDF are available that
    support raster-image manipulation and display and browsing through
    multidimensional scientific data. The HDF software includes a package of
    routines for accessing each HDF data type, as well as a lower-level
    interface for building packages to support new types. HDF supports both C
    and Fortran interfaces, and it has been successfully ported to a wide
    variety of machine architectures and operating systems. HDF emphasizes a
    single common format for data, on which many interfaces can be built.

    NCSA has implemented software that provides a netCDF interface to
    HDF. With this software, it is possible to use the netCDF calling
    interface to place data into an HDF file. The netCDF calling interface
    has not changed and netCDF files stored in XDR format are readable, so
    existing programs and data will still be usable (although programs will
    need to be relinked to the new library). There is currently no support
    for the mixing of HDF and netCDF structures. For example, a raster image
    can exist in the same file as a netCDF object, but you have to use the
    Raster Image interface to read the image and the netCDF interface to read
    the netCDF object. The other HDF interfaces are currently being modified
    to allow multi-file access, closer integration with the netCDF interface
    will probably be delayed until the end of that project.

    Eventually, it may be possible to integrate netCDF objects with the rest
    of the HDF tool suite. Such an integration will then allow tools written
    for netCDF and tools written for HDF to both interact intelligently with
    the new data files.

We met with the HDF developers a couple of years ago to see if some sort of
synthesis or combination would be desirable and practical.  What came out of
that was NCSA's decision to try to add the netCDF interface to the set of
interfaces layered on top of the HDF format.  HDF originally had a simpler
interface for scientific data, but the NCSA developers liked the netCDF
interface better.  

There are significant performance differences between the two
implementations of the netCDF interface, with each significantly faster than
the other for some kinds of operations.

HDF emphasizes a single common format for data, on which many interfaces can
be built.  NetCDF emphasizes a single common interface to data, implemented
on top of an architecture-independent representation.

__________________________________________________________________________
                      
Russ Rew                                              UCAR Unidata Program
address@hidden                                        P.O. Box 3000
(303)497-8645                                 Boulder, Colorado 80307-3000