HFWG Recommendation R8:

    On the use of the HDUCLASn keywords to provide a hierarchical classification scheme for FITS extensions

    (Approved: 1994 Feb 02)

    It is usually necessary not only to use a well-defined FITS format to store a given dataset, but also to INFORM a reader (either human or software) that such a defined format is in use along with details on the specific version of the format, detailed contents, etc.

    The following keywords are recommended for such purposes in each extension:

    • HDUCLASS - a character string giving a general identifier of data format used
    • HDUDOC - a character string giving the document (preferably published) that describes the format/classification used
    • HDUVERS - a character string giving the specific version of the document specified by HDUDOC.
    • HDUCLASn - an indexed set of character strings giving the classification of data in the extension

    Comments on Usage:

      HDUCLASn is an indexed keyword. HDUCLAS1 should provide the general data classification. HDUCLAS2-HDUCLAS9 specify subclasses within the general classification. Such subclasses can specify either modifications to the general classification or modifications to the data contained in the file.

      The current list of HFWG-approved values for HDUCLASn is provided in HTML and as an ascii list in the document ftp://heasarc.gsfc.nasa.gov/fits_info/ofwg_recomm/hduclas.list

      We propose that the value of HDUVERS have the form X.Y.Z, where X, Y, and Z are integers, with the following understanding:

      • change in X: format has been completely redefined - software take note
      • change in Y: bug fix or new keyword(s) added which may impact software
      • change in Z: minor change, no impact on software

      Of course, designers of data files have the option of using an existing format or defining a new format. Should it be necessary to define a new file format, the file designer(s) should provide the values of HDUCLASS, HDUDOC, HDUVERS and HDUCLASn keywords appropriate to that format if it is desired that the format be adopted as a standard. Each value of HDUCLASS must be unique; the OGIP volunteers to keep a list of all values of HDUCLASS used.


        HDUCLASS = 'OGIP   '            /  Conforms to HEASARC/OGIP conventions
        HDUDOC   = 'Arnaud et al. 1992, Legacy 2, p 65.' / format specification
        HDUVERS = '1.0.0   '           / version of format
        HDUCLAS1= 'SPECTRUM'           / spectral data format
        HDUCLAS2= 'BACKGROUND'         / background spectrum