[fitsbits] start of Public Comment Period on the Green Bank convention

[Mark Calabretta]

On Fri, 19 Jun 2015 13:34:26 +0200 (CEST)
Lucio Chiappetti <lucio at lambrate.inaf.it> wrote:

Hi Lucio,

> The proposed draft text is available at 
> http://sax.iasf-milano.inaf.it/~lucio/FITS/Conventions/greenbank-upd2.pdf

The Green Bank convention applies to most keywords in tables, not just
the WCS keywords, so belongs in Chapter 7, not 8.  Perhaps make a new
section, 7.4.

Clearly, there are many keywords that the Green Bank convention cannot
apply to: EXTNAME, EXTVER, EXTLEVEL, XTENSION, BITPIX, NAXIS, NAXIS1,
NAXIS2, PCOUNT, GCOUNT, TFIELDS, EXTVER, TFORMn, TTYPEn, TUNIT, TDIMn,
THEAP, and probably others.  As a standards document, they all have to
be identified and listed.

The statement on p29

  It is *strongly recommended* that if the value of a TTYPEn keyword is
  the same as the name of a reserved keyword, than the quantity in that
  field should conform to the definition of that keyword.

should be changed to

  If a TTYPEn keyvalue is the same as the name of a reserved keyword
  not on the list of exclusions [as above], then [not "than"] the
  quantity in that field *shall* be interpreted according to the
  definition of that keyword.

because what other sense can software make of it?  Or even a human
reader for that matter. 

There should be a statement regarding precedence if a keyword and column
of the same name are both present.

There should be some mention of the fact that columns can only be
collapsed if the column name can become a legitimate FITS keyword.

Sect. 8.2.1 should retain the comments about TFORMn = 'CRVALia' being
allowed for historical compatibility, though that usage should be
deprecated.

FITS readers normally only need to parse keywords and read keyvalues.
Implementing the Green Bank convention requires that they also parse
keyvalues associated with TTYPEn.  Unless FITS readers know in advance
whether the Green Bank convention is or is not in use, they will have
to parse TTYPEn keyvalues for *every* table extension they read.  This
leads me to suggest that there ought to be some standard way to signal
that the Green Bank convention is in use.  For SDFITS, this is implicit
in EXTNAME = 'SDFITS'.

Regards,
Mark Calabretta