# VOunits draft

Rob Seaman seaman at noao.edu
Tue May 26 08:40:45 PDT 2009

```Hi Hervé,

> On my side, I don't understand why you're focusing on these two ends
> of the distribution !

I'm just trying to write the email that gets me out of the
conversation :-)

I'm mentioned yotta and yocto because they are part of SI and it keeps
being asserted that we must support ALL of SI.  It seems more
important to support table columns in units of parsecs (something we
definitely will want to do) rather than columns in units of yoctohertz
(something I am skeptical will prove of practical value).

> But will lead to revision in the future if new cases appear where
> new prefixes are needed.

As with other VO discussions about maintaining enumerated lists (eg,
UCDs, vocabularies), there must always be a process for revision.
Astronomers frequently (meaning several times each century) create new
units.

> the basic idea (that comes from the best practise of numerical
> computation community)

Is there a URL pointing to a document discussing the topic of
numerical precision in the context of SI prefixes?

> is the fact we can separated the magnitude (10^n here, not the
> astronomical one :-) ) from the significant part of a real number
> (mantissa). By chance (?), the SI use only power of ten to encode
> unities...
>
> Let's assume a parcsec to be (I know this is an approx) :
>   1 pc =  3 085 677 581 282 10^{16} m
>   ...

Thanks for the discussion of floating point precision.  My two
comments are that first, conversions may only be needed if the VO
standards require them.  There are lots of tables containing columns
of distances in parsec (and kilo and mega parsecs).  Are we going to
require that these be converted to meters?

1 Parsec = 3.08568025 × 10^16 meters (according to wikipedia)

So will folks have to convert 1 megaparsec to 30.9 Zettameters (Zm)?
Bear in mind that the distance estimate may be accurate only to +/-
50% and may scale with the hubble constant...

> IVOA standards already depend heavily on external standards as W3C
> ones for instance. W3C seems to be such an external authority. FITS
> is only heavily used, that is an IAU standard.

I'm not adverse to using standards.  The question is whether we are
choosing standards solely because some authority has blessed them.

> Maybe I'm wrong, but what I've understood from the discussion is
> that the defintion of a common unit system is needed for 'internal'
> purpose (eg service-to-service communication). At the level of the
> human client, any 'traditional' unit is allowed. Am I right?

The choice of internal units must still make sense astronomically and
computationally.  But I think that VO facilities must also be
considered to include their user interfaces.  If our internal systems
are required to use zettameters and our external interfaces parsecs,
some agent must perform the conversion.

Rob

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