[QUANTITY] and [SIMPLE]: Forking the discussion

[David Berry]

Jonathan,
          For some reason I missed this one in my mail box - I've just
seen it now on www.ivoa.net:

> At Cambridge and Strasbourg we identified two such layers,
> OBSERVATION to describe a whole dataset, with the understanding that
> must include the example of a 2D sky image, and QUANTITY to describe
> numbers and arrays associated with a physical concept, to be used
> as a building block for OBSERVATION. My touchstone for QUANTITY is
> that it should contain everything that is associated with a single
> astronomical/physical concept, which is why I'm happy to include
> units and errors but nervous about including mappings ("arguments"
> to quantity in terms of the earlier discussion). Brian has just
> argued that in fact the mappings problem is not separable because
> it will affect the rest of Quantity; I'd be interested to hear
> his specific arguments on this.

One argument could be consistency. The simplest model you could have
would be a data structure such as simple n-D array with *no* extra
meta-data. Each element of such an array has only two things of
significance:

1) its value
2) its position within the array

Once you have specified these 2 things, you have said everything there is
to say about any specific array element.

One step up in complexity from this could be to add meta-data describing
the nature of the array values (item 1). This would no doubt include units
but may include other things as well (for instance if the array contained
frequency values, you should really say what the rest frame is, etc).

But if we add meta-data to describe the values, is it not being
inconsistent if we do not also add meta data describing the positions?
It seems odd for the Quantity to say "I'll tell you what the data
values mean, but I won't tell you what the data positions mean".

David






----------------------------------------------------------------------
Dr David S. Berry    (dsb at ast.man.ac.uk)

STARLINK project		 |	Centre for Astrophysics
(http://www.starlink.ac.uk/)	 |	University of Central Lancashire
Rutherford Appleton Laboratory	 |	PRESTON
DIDCOT				 |	United Kingdom
United Kingdom			 |	PR1 2HE
OX11 0QX