[fitswcs] double, double, toil and trouble

[Arnold Rots]

We'll make it clear that, if one wants a floating point solution and
doubles don't provide the required precision, one may use the doublet
of doubles; and if one does, this is the way to use them.
That probably requires a few sentences on alternative solutions, like
the ones you mention.

  - Arnold


Rob Seaman wrote:
[ Charset UTF-8 unsupported, converting... ]
> > 3.1.4. Higher Precision in Binary Tables:  Doublet Vectors
> > In binary tables one may use vector pairs. The time column in
> > such a table shall contain a vector of two doubles where the ?rst
> > component of the doublet contains the integer portion of the time
> > value and the second one the fractional part.
> 
> This is confusing two issues, the precision required with the desired  
> representation.  Two doubles (or a single quad floating point) may be  
> sufficient to a particular purpose - they are not necessary.  Similar  
> precision can be obtained by the prudent use of integer or fixed point  
> types.
> 
> The workflow logistics of various use cases may benefit from avoiding  
> floating point.  For instance, integer types compress much better.  In  
> general a time series might "want" to be expressed as delta times  
> rather than quasi-absolute times.  (As the document points out, all  
> timekeeping is relative.)
> 
> My concern is just that such usage as described in section pi (3.1.4)  
> ought be permitted by FITS, not required.
> 
> Rob
> 
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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