[QUANTITY] Do we need it? Some quantities

[martin hill]

Being a bit radical here, but what is quantity for?  I've read the list (honest)
but it strikes me that really a quantity is simply a number.  A range for
example presumably consists of two quantities.  So perhaps we should start
further back and look at what kinds of quantities we want to describe -
personally I suspect we'll end up with a bunch of different 'quantity models'
some of which are not necessarily related...  

So as a non-astronomer, we might have:

Value: simple number (integer or real)

Error: 
   simple (+/- absolute)
   percentage (+/- percentage)
   normalised* (sigma?)
 
Range: Min Value + Max Value
   might have errors attached to range or to values?

Value & Range might have a unit attached, but only a simple error will.  

*not sure of the right term here

Including quality is a scary topic (for me as an implementer!). Can we make it a
separate [Quality] model altogether?  It will get attached to all kinds of
things, including presumably complete data sets?

Martin

Quoting Brian Thomas <brian.thomas at gsfc.nasa.gov>:

> 
>         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
> 
> 
> 
> 


-- 
Martin Hill
07901 55 24 66
www.mchill.net