[QUANTITY] Attempt at gathering consensus view of quantities through requirements capture

[Brian Thomas]

        Hi all,

        I came away from the ADASS/IVOA meeting being encouraged that some
        progress could be made at data modeling as long as we tried to side step
        semantic issues of naming and coding specifics, and instead started at
        the most general level of needs (or requirements). This means no attempts
        to produce "the" UML diagram with all specifics and trying to hammer each
        of our models home. The details in each are too much for the others to
        generally comprehend, or even, in general, want to spend time studying.

        Hence, I propose that we attempt to gather requirements for the quantity, to
        better define what it is we all want. These requirements would be *then* used
        to create a consensus UML diagram *later*. I would hope that for this discussion,
        we can just debate the requirements of "quantity", perhaps with points illustrated
        via a UML diagram or a use-case, but no "overall" UML diagram being produced
        until we all largely feel the "fundamental" requirements have been generally
        agreed to.

        If this experiment is successful, I would hope that the DM group will
        recommend a draft of these requirements as part of the greater "Observation"
        whitepaper that I understand Jonathan wants to prepare, or perhaps as a
        separate IVOA note.

        So, to start the ball rolling, here's some requirements that I saw
        in the various models (all requirements start with the "="), and/or popped
        up the the informal discussion at IVOA/ADASS. I have attempted
        to arrange these in order of general acceptance, rather than "importance":

<requirements>


   = Quantity is a container class that holds scientific, engineering information.

   = Components of the observation model will inherit, as needed, from the quantity.

   = The quantity has an associated class, "coordinate transform/mapping", which is
     used to transform one quantity into another (dimensionality of the two quantities
     is the same).

   = The information in quantities, in order to make it valuable and machine readable
     requires that it be described by meta-data that include type of units, type of data
     format (such as "long", "float", "string", etc) and its accuracy (which includes
     things like "quality" flags and "statistical/systematic errors").

   = The quantity may be multi-dimensional (this is _almost_ a universal feature of extant models).

   = The quantity may hold information which comprises scalars, vectors or other quantities (tuples).

   = Primary/top requirement: The quantity will be used to facilitate the search,
     exchange and data fusion in the VO. [The "data fusion" part leads to the 3rd requirement
     in the list]

   = The quantity will be able to completely describe all the information (data or
     meta-data) in a FITS file or VOTable.

        which (I think) leads to..

   = The quantity is a container class for _meta-data_ as well as _data_.

</requirements>


        So there is a "starting" list. Nothing "official", but perhaps it can lead to
        some consensus document that is.

        Thats all for now,

        Regards,

        =b.t.

-- 

  * Dr. Brian Thomas 

  * Code 630.1 
  * Goddard Space Flight Center NASA

  *   fax: (301) 286-1775
  * phone: (301) 286-6128