Re: HDF

> Organization: NCAR/CGD
> Keywords: 199406091630.AA07008

Hi Dennis,

> I have been asked to write a one page
> "position paper" for our group, the
> CLimate Analysis Section, for consideration
> by the people who will decide which format
> to use for future archiving by the climate
> models.
> 
> I will let u see  it before I send it it.
> I will be simple. As I stated in my previous email
> it is between GRIB and netCDF. Howwver, I would like
> to say a few things about HDF. I have asked Dennis
> Joseph about HDF and he says his knowledge of that
> format is rudimentary. Do u know where I can get
> a "non-technical" overview of HDF? In a nutshell,
> what do u think of HDF? pros-ncons?

Yes, below is a recent answer to a very similar question from Ken Schroder
at BB&N.

(Incidentally, you can find this and other information about HDF and netCDF
 by using Mosaic.  Just look at the netCDF home page at
 http://www.unidata.ucar.edu/packages/netcdf/.  Then enter "hdf" in the
 first searchable index of answers to netCDF support questions.  When you
 hit return, you get a menu of all previous support answers to netCDF
 questions that contain the word HDF, sorted by the messages that contain
 the most occurrences first.  Just click on the first of these and you get
 the answer I've appended below.  It doesn't always work this easily, but it
 can be a useful way to get information that doesn't appear in formal
 documents.)

------------------------------------------------------------------------------

  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.

There is a WWW page containing an overview of HDF and links to lots more
information about it.  Just point your Mosaic or other WWW server at:

        http://yahoo.ncsa.uiuc.edu:8001/

(The yahoo host is often overloaded, so you will have to be patient or lucky to
get the above link to work.)

Please let me know if you have additional questions about HDF and netCDF.

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