[fitsbits] Extended Comment Period on 2 FITS Proposals

[Bob Garwood]

William Pence wrote:

>
> 2.  A proposal to incorporate 2 unofficial appendices into the 
> official FITS
> Standard.   These appendices describe the TDIMn keyword convention for
> specifying the dimensionality of vector columns in binary tables and the
> variable length array convention.  The original announcement regarding
> this proposal can be found at
> http://listmgr.cv.nrao.edu/pipermail/fitsbits/2004-October/001519.html
>
I've been meaning to make a couple of suggestions to this proposal.

Even with the modified text, the TDIM keyword isn't particularly useful 
for a variable-length array column using the heap.  The suggested 
replaced wording still implies an equality between the total number of 
elements implied by TDIM and the
length specified by the variable length array descriptor.  Since the 
whole point of using a variable-length array column is so that you can 
vary the length of the array associated with that column, it seems to me 
that there needs to be some statement as to how a TDIM keyword is to be 
interpreted in that case (when the number of actual elements in the 
array is < the total number implied by TDIM). 

I think it would be more useful in that case if the TDIM values for that 
column could be expressed as a column itself instead of a keyword.  e.g. 
if the variable column in question is column 05 then somewhere you'd 
have another column with a TTYPE value of "TDIM05". 

Secondly, I think the above is useful even in the case where 
variable-length arrays aren't used.  If you have nearly the same sized 
arrays to store in a column, you could pad them all out to the same size 
and store them without wasting too much space.  In that case, it would 
be useful to mearly require that the number of elements implied by any 
TDIM in the column be <= the element count for the column it refers to 
and you'd have an associated column of TDIMs that would describe the 
specific shape of each cell.

Bob Garwood