[fitsbits] start of Public Comment Period on CONTINUE Long Kwd convention

[Mark Calabretta]

On Fri, 26 Jun 2015 12:19:37 +0200 (CEST)
Lucio Chiappetti <lucio at lambrate.inaf.it> wrote:

Hi Lucio,

> About the word "convention" we should not be overly picky. The FITS 
> standard has been growing out of accepting conventions. There are various 
> items in the current (3.0) standard document [make an acroread search !] 
> which are ALREADY CALLED conventions (Random Groups, Variable Arrays, 
> unsigned integers, all the WCS stuff ... perhaps even the Y2K agreement).
> 
> So we have simple conventions (legal FITS constructs used only inside 
> specific projects or subcommunities), registered conventions (documented 
> in an accepted/endorsed way, useful to AND USED BY several subcommunities 
> and proposed as examples "if you need to do X why not do like this ?") and 
> conventions which are parts of the standard, while remaining optional ("IF 
> you need to do X then you SHALL do it this way")

Standards documents need to be clear, precise, and unambiguous.

FITS 3.0 often deliberately distinguishes between conventions and
standards, sometimes explicitly so, e.g. p28:

 "Any coordinate type that is not covered by this standard or an
  officially recognized FITS convention shall be taken to be linear."

The fact is that most instances of the word "convention" are correctly
used.  

Noting that "conventional coordinates" have a particular meaning in
FITS WCS, the few incorrect usages ought to be fixed, as follows:

- Table 1: three instances of "Adoped conventions" should be replaced by
  "Adopted proposals".

- p4, "random groups convention" should be "random groups construct".
  Likewise p10 and p42.

- p8, "IAU convention" should be "IAU guidelines".

- p27, "There are three conventions" is plain wrong.  It should be
  "There are two standard constructs and one historical convention".
  
  Likewise, "The third convention was widely used before the development
  of the two previously described conventions" should be "The CROTAn
  convention was widely used before the development of the two previously
  described standard constructs".
  
  "the CDi_j keyword convention" could be correct if the reference is
  to software that supported it before incorporation into the standard.

- p28, "naming convention" -> "naming scheme"

- p31, "Following the convention for the CTYPEia keyword" should be
  "Following the standard construct for the CTYPEia keyword".

That's it.  Every other use of the word "convention" or "conventional"
is in accord with the generally accepted meaning that distinguishes
conventions from standards.

Also, I must point out that the simplest legal FITS file consists of
SIMPLE, BITPIX, NAXIS and END keyrecords, followed by 32 blank
keyrecords.  Everything else in the standard is optional.  "Optional"
has no significance whatsoever.  

> while current 
> registered conventions are already legal FITS, harmless if ignored,

Ignoring INHERIT could lose you a stack of porentially important
keywords.  Ignoring CONTINUE could lose the nether end of string
keyvalues.  Ignoring tiled compression could render the image
unreadable.  How is that harmless?

Regards,
Mark Calabretta