revised quantity draft: comments from Aus-VO

[Anita Richards]

>
> 3. [suggestion] the question of accuracy is raised, and in 4.13 a tuple
>    is given as a possibility for non-simple data types.  Looking at all
>    astrometry packages, times are already always given as two doubles
>    (you can barely get a us precision over a full year in one word)
>    and the same is getting true for frequencies: THz central frequency
>    with precise offsets.  A data model where it is possible to specify
>    an offset in a separate word of a tuple would solve this (and future)
>    precision issues.

I have already run into rounding errors where applications use decimal
radians and positions need to be expressed to milli-arcsec accuracy.  In
fact I am told that if full double precision is used properly then it
should be OK for micro-arcsec.  However, in practice that is almost never
the case, in my experience; even applications which handle data correctly
internally don't report it correctly (AIPS, Aladin... that is just the
ones beginning with A...).  Using a tuple can make things worse
in that you can be returned the sum of two parts of a tuple which have
both been rounded separately.  The other extreme is applications which
return a very large No of decimal places regardless of the accuracy,
convert integers to non-integers etc.

mm-vlbi results are already being reported with a precision of ~4e-13
radians (sub-uas) and of course you need an extra place for rounding.
Pulsar timing and SETI use the sort of precision David mentioned in the
time/frequency domain.  There are probably many other examples.  This is
not an academic issue.

So I agree strongly with David that we need to ensure that accuracy is
properly handled _and returned_ and the suggestion he makes is probably
the best way to do it (I am happy to leave it up to experts) but I wonder
how best to get good practice _adopted_, i.e. to produce applications and
models which work to the appropriate degree of accuracy and which leave
room for the sort of very high accuracy which is in use - not to mention
the future.

Sorry to labour the point, but I did some work briefly with Al Stirling on
investigating how VO tools which currently exist might cope with ALMA data
and we found that most of them can't even cope with 22 GHz MERLIN or 1.6
GHz VLBI data (i.e. milli-arcsec accuracy), which is actually a severe
handicap here and now.

cheers
a


- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Dr. Anita M. S. Richards, AVO Astronomer
MERLIN/VLBI National Facility, University of Manchester,
Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K.
tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).

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