[fitsbits] Start of the 'INHERIT' Public Comment Period

Rob says:

On 4/5/07 6:57 PM, "Rob Seaman" wrote:

Bob says:

I have to express some concern about registering the INHERIT
convention.
The documentation notes a number of potential problems that can occur
when FITS software that is unaware of the convention is used to read,
interpret, and write new copies of files that use INHERIT.

1) Isn't the notion of the convention registry to record what is already
being used around the FITS community? NOAO relies on INHERIT
for our MEF data from the MOSAIC DHS and will rely on it for the
NEWFIRM DHS. The NOAO Pipeline understands INHERIT.
The convention has served us well.

It is clear it is in use. STScI uses it for HST data, too.

2) NOT documenting INHERIT certainly won't help the community.

3) IRAF has understood INHERIT for three-score-and-ten dog years.

Well, I doubt it is THAT long....

It would be cleaner, I think, to define more clearly the rules for
how primary
headers pertain to extension headers (e.g., the concept of inheritance
applies by default, or whatever).

4) Yes, but that boat has sailed. The community has been on a course to
deal with inheritance since this note from the image extension paper:

"Although allowed, it is recommended that the primary header does
not set the keyword NAXIS=0, since it would not make sense to extend a
non-existing image with another image."

FITS is either going to tie the contents of separate HDUs together
semantically or not. The community eagerly - and widely - adopted the
notion of the primacy of the primary HDU - likely before the words above
were published. Implicit here is that the primary header of an empty
HDU is often used for information that applies to the entire file.

That would be my interpretation, too, but as the INHERIT document notes,
this was not made explicit.

5) If not INHERIT, then what?

Making the rules explicit. The FITS review panel should look at this and
see if it would be a clarification to existing practice, or something new
and potentially standard-breaking.

6) And we'd still be left with gazillions of files that rely on this
convention
as an organizing semantic principle. Clearly the first step in
revisiting the
fundamental semantics of a FITS file (of which keyword inheritance is
only
a small part) would be to protect our investment in previous data
products
by documenting the current de facto standards.

6a) In any event, the first step in deprecating any convention would be
to recognize its existence.

I did write that, Rob....

" If we are to include INHERIT in the FITS registry, we should perhaps do so
solely to document the practice."

This is something we might recommend to the recently formed FITS
review
panel to discuss.

7) By all means, but only in an advisory capacity. I presume we're
not thinking
of changing the fundamental FITS standards process? It has served us
well
for many years.

No such radical suggestion. It is like the panel that I chaired some ten
years ago. The is to look for ambiguities, inconsistencies, etc., and tidy
them up. There is no idea of circumventing the process.

8) Really - isn't documenting the current usage the simplest thing to
do?
All of these conventions are conventional, rather than standard,
precisely
because they reflect issues that were thorny to deal with the first
time around.
Few will fall into the same category as the checksum keywords - i.e.,
pre-existing legal FITS usage demanding no clarification of the
standard.

Rob

The thing I am concerned about is conveying a sense of "this is a great
idea" by registering the convention. This one in particular, given the
caveats written into the document itself, requires one to pause. The
language describing registered conventions says

" These conventions are not necessarily endorsed by the IAU FITS Working
Group."

But that is pretty weak, so what I am suggesting is that this review for
this extension might say that the FITS WG notes its existence, and provides
documentation, but does not encourage further use. Or that potential
adopters fully understand the potential problems. Something like that.

Bob