[fitsbits] reopening of Public Comment Period on the CONTINUE convention

[William Pence]

On 3/10/2016 11:20 AM, Tom McGlynn (NASA/GSFC Code 660.1) wrote:
> Two further nits...
>
> I am unclear of the legality of the use of CONTINUE keywords in a
>      CONTINUE = '&'  / A continuation character.
> context.
> The new convention is described before the general constraint in 4.4.2
> reserving the use of the keywords defined later in the
> standard, but they are mentioned afterwards, so I think that status of
> other uses of CONTINUE is a bit hazy.  [The use of
> CONTINUE as a comment style keyword is explicitly allowed though.]

Huh?  It is not clear to me what your question is.  If you are in 
particular referring to the statement that an orphaned CONTINUE keyword 
should be interpreted as containing commentary text, that is simply a 
restatement of the definition in section 4.1.2.3 that if any keyword 
does not have a value indicator (i.e., the equals sign and space 
character in bytes 9 and 10) then bytes 9 through 80 on that keyword 
should normally be interpreted as commentary text.  So the 
interpretation of orphaned CONTINUE keywords is not unique, and a 
similar interpretation applies to any other user-defined keyword that 
does not have a value indicator.

> If someone has used CONTINUE as a keyword, have their files become illegal?
> I don't actually care what the answer to this is, but I think it should
> be clarified either way.

No. Section 3.7 unequivocally states that any valid FITS file shall 
remain a valid FITS file at all future times.  It is because of this 
'Once FITS, always FITS' rule that new requirements are rarely added to 
the standard and then only when the impact on existing FITS files is 
considered to be minimal.  In particular, When new reserved keywords are 
added to the Standard  (which has happened several times over the 
years), I think it is generally understood that any existing FITS files 
that may have used that keyword in a different way are exempt from the 
new requirements on how that keyword should be used.

>
> The other nit is the new final sentence in 4.2.1.1 is puzzling.
> I.e., what does
>    In general, no length limit less than 68 is implied for
>    character-valued keywords.
> mean?  We'd just made it clear that single record character string values
> can be up to 68 characters.  I don't know that the this following
> sentence  does any harm, but I can't see what
> work it is doing and it may confuse others beyond me.  In fact, there is
> no length limit whatsover on string values now.

The highlighting of that entire paragraph as being new is an editorial 
mistake.  The only thing that is actually new in that paragraph is the 
phrase "that can be represented on a single keyword record".  Otherwise, 
that same wording can be found in previous versions of the FITS Standard 
document going back to the 2.0 version in 1999.  The sentence stating 
that "no length limit less than 68 is implied" was added back then to 
refute a statement in the original Wells et al. FITS paper which implied 
that character string keyword values could normally be assumed to be 8 
characters in length, or at least that the value could be uniquely 
defined by only reading the first 8 characters.  Recall that that 
original FITS paper was written in 1980 when the amount of available RAM 
was usually measured in units of kilobytes.

>      Tom
>
>
>
>      Tom
>
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-- 
____________________________________________________________________
Dr. William Pence    Astrophysicist     William.Pence at nasa.gov
NASA/GSFC Code 662     [Emeritus]       +1-301-286-4599 (voice)
Greenbelt MD 20771                      +1-301-286-1684 (fax)