[fitsbits] FITS Binary Table Proposals

[William Pence]

I've made most of the changes suggested by Tom (you may need to clear your 
browser's cache to force it to download the new version dated 12 November):

Thomas McGlynn wrote:
> They look pretty good to me, but here are a few comments.
> 
> Some tiny editorial points in the TDIM convention.
> I'd just make it
>    8.3.6 Multidimensional Arrays
> This is now the standard way to define multidimensional arrays in FITS 
> binary tables.

Done.

> For the second sentence
>    Any column with a dimensionality ... has ...
> not "will have" since we use present tense everywhere else in the 
> discussion.

Not done.  The presence of the 'will' adds some desireable emphasis.  If the 
column contains a multidimentional array, then it 'will' (meaning 'must') 
have the TDIM keyword to indicate this fact.

> Typo in "see Section[m] 8.3.5"

Corrected.

> Similary for the Variable length arrays...
> 
>    8.3.5 Variable Length Arrays

Done.

> I'm not sure that the last sentence to the third paragraph means
> anything, i.e., "Other established..." so I'd get rid
> of it.

It means that other FITS conventions, such as how to define world 
coordinates, that have been defined to apply to fixed length arrays can also 
be applied to variable length arrays.

> There is an upper limit to the size of the stored array: the maximum
> value of a 4 byte integer

A comment to this effect has been added

> A few things that should probably be tied down specifically that are
> currently a bit loose:
> 
> 1. Are the sizes and offsets signed integers or unsigned?
> If there were defined to be unsigned integers they could handle
> objects twice as large and we wouldn't need to worry about
> negative offsets.

I don't think there is any question that these are intended to be signed 
integers, so I added the adjective 'signed'

> 2. Can THEAP be negative?   The possibility of a negative 'gap'
> is not explicitly forbidden, but probably should be.   Or
> do we want to allow reuse of data from the table as variable
> length data.  One could make interesting structures...  Even if
> we permit negative THEAP's (which I suspect will not be the case)
> presumably it needs to be restricted to staying within the
> current Data unit.

If THEAP is negative, it would be pointing a location before the start of 
the main data table, i.e., to somewhere in the Binary table header, or even 
to a previous extension in the FITS file.  It would be sensible to require 
that THEAP be positive, but that will require a change to a different 
section of the FITS Standard than what is being considered in this proposal.

>         Tom McGlynn
> 
> William Pence wrote:
> 
>> In response to the comments received so far during the official Public 
>> Comment Period on the proposals to incorporate the TDIMn and variable 
>> length array conventions for FITS binary tables into the official FITS 
>> Standard, I have created new detailed drafts of the proposals which 
>> are available at
>>
>> http://fits.gsfc.nasa.gov/bintable_proposals.html
>>
>> In particular, these new drafts contain the following changes:
>>
>> 1. As recommended by Tom McGlynn, the drafts now specify precisely the 
>> wording that should be inserted into the FITS Standard.
>>
>> 2. As recommended by Bob Garwood, the TDIMn convention has been 
>> extended slightly to allow the size of the array as given by the 
>> product of the dimensions to be "less than or equal" to the allocated 
>> array size in the FITS file.
>>
>> 3. As recommended by Francois Ochsenbein, a cautionary phrase has been 
>> added to warn users that the variable length array convention may 
>> present difficulties for applications that ingest FITS files via a 
>> sequential input stream.
>>
>> The wording of these proposals may still not be optimal, so any 
>> further suggestions are welcome.
>>
>>
>> Bill Pence
> 
> 
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Bill Pence
-- 
____________________________________________________________________
Dr. William Pence                          William.D.Pence at nasa.gov
NASA/GSFC Code 662         HEASARC         +1-301-286-4599 (voice)
Greenbelt MD 20771                         +1-301-286-1684 (fax)