[fitsbits] start of Public Comment Period on the column limits convention

[Mark Calabretta]

On Fri, 19 Jun 2015 13:24:11 +0200 (CEST)
Lucio Chiappetti <lucio at lambrate.inaf.it> wrote:

> This is to announce the official start of a 3-week formal Public Comment 
> Period on the incorporation of colum limit (aka TLMIN TLMAX) convention in 
> the FITS Standard.

This proposal seems uncontroversial to me, though I have several
suggestions concerning the documentation.

- As they are specific to tables, TDMINn/MAXn & TLMINn/MAXn belong in
  Chapter 7, not 4, specifically, in Sects. 7.2.2 and repeated in 7.3.2.
  
  (I also suggest that the current misleading title of Sect. 4.4.2.6 be
  changed to "HDU labelling keywords".)

- In the following sentence

    Any undefined elements in the column (or any other IEEE special
    values in the case of floating point columns in binary tables)
    *shall* be excluded when determining the value of these keywords.

  if the intention of the reference to IEEE special values is to exclude
  plus and minus infinity, then that should be stated explicitly, and
  some sort of explanation given.

- In the following statements

  1. TDMINn Keyword.  The value field *shall* contain a number giving
     the minimum physical value actually contained in column n of the
     table.

  2. TDMAXn Keyword.  The value field *shall* contain a number giving
     the maximum physical value actually contained in column n of the
     table.

  3. If the value of TDMINn is greater than TDMAXn, ..., then the values
     of the pair of keywords *should* be interpreted as undefined.

  from (1) and (2), which are imperative, it follows that
  TDMINn <= TDMAXn imperatively, so TDMINn > TDMAXn can only happen by
  mistake.
  
  Surely the standard needn't elaborate on the infinity of mistakes
  that FITS writers might make.  Anyway, in such a case, "*should* be
  interpreted" ought to be "*shall* be interpreted".
  
  The same comment applies for TLMINn and TLMAXn.

- Instead of saying
  
    the minimum physical value actually contained in column n

  why not just say

    the minimum value in column n

- Given the two statements

  1. TLMINn Keyword.  The value field shall contain a number giving the
     minimum legally defined physical value that may be contained in
     column n of the table.

  2. TLMAXn Keyword.  The value field shall contain a number giving the
     maximum legally defined physical value that may be contained in
     column n of the table.

  logically it follows that a value outside the range [TLMINn,TLMAXn]
  must be illegal, which is not what the following says:

  3. It is permissible to have values in a column that are less than
     TLMINn or greater than TLMAXn, however, the interpretation of any
     such out-of-range column elements is not defined.

  Instead, I imagine it should be

     Column values outside the range TLMINn to TLMAXn *shall* be
     interpreted as undefined.

- Instead of saying

    the minimum legally defined physical value that may be contained in
    column n

  why not just say

    the legal minimum for a value in column n

- I guess if one or other of TLMINn and TLMAXn are omitted, the other
  defaults to plus or minus infinity.


Regards,
Mark Calabretta