Clive Page wrote:
> On Wed, 21 Jun 2006, Mark Taylor wrote:
>
>> 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?
>
> In the world of relational databases there have long been disputes over
> how many different types of null should be supported. Since the
> set-theory basis of RDBMS actually becomes inconsistent in the face of
> even one type of null, there are many who think that they are all evil.
>
> In practice, as Mark said astronomers have long managed with just one
> type of null in FITS files. Once the idea of having more than one type
> of null gets around it will be very hard to get agreement on where to
> stop, and I have seen little evidence of need for more than one. I
> think it would be more productive to work on a standard way of
> expressing the notion of upper (or lower) limits - something which is so
> far missing from most astronomical formats.
>
It is perhaps worth mentioning that FITS allows all the IEEE special values
in 32-bit and 64-bit floating point data. There are more than 16 million
different 32-bit NaN values (if I calculated correctly) and a vastly larger
number of 64-bit NaN values. In principle, FITS software could use
different NaN values for different purposes, but as far as I know this has
never been done. FITS also supports the IEEE special values for positive
and negative versions of denormalized numbers, underflow, overflow, and
infinity, but in practice I don't think that these have ever been
deliberately used in FITS files. A chart of all these special values
can be
seen in appendix H of the FITS Standard
(http://archive.stsci.edu/fits/fits_standard/node95.html
Bill Pence
-- ____________________________________________________________________ Dr. William Pence pence-at-milkyway.gsfc.nasa.gov NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice) Greenbelt MD 20771 +1-301-286-1684 (fax)Received on 2006-06-22Z09:18:28