Empty TD element wording

[Mark Taylor]

On Tue, 20 Jun 2006, Doug Tody wrote:

> It would be good to make a distinction between a numerically indefinite
> value (NaN for floating point) and no value (undefined, NIL, NULL,
> etc.).  I think what is being talked about here is an "undefined"
> value, i.e., an element for which no value has been assigned.  Even for
> floating data, something other than NaN is needed to specify an
> undefined value.

I can see that this can be justified in theoretical terms, but I must
admit I've never thought much about the distinction and any software 
I've written has got on fine by using NaN (or a magic integer value) 
to cover both undefined and numerically indefinite.  FITS BINTABLE
and IMAGE seem to have managed without the distinction as well.
Are there really compelling scenarios for this requirement?

>                   Since VOTable is a text format, the best way to do
> this is probably with another magic string value of some sort, rather
> than reserve a valid numeric value.  - Doug

VOTable is only a text format in its TABLEDATA variant; as per my
previous message a magic string wouldn't work for its BINARY or FITS 
variants.

-- 
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-928-8776 http://www.star.bris.ac.uk/~mbt/