VOunits draft

[Francois Ochsenbein]

Hi Bill,

Just a couple of comments:

-- vs cgs: the IAU recommends the usage of SI since 20years now,
   and lists especially erg, gauss, etc as "units which use is
   deprecated" (see the url
   http://www.iau.org/science/publications/proceedings_rules/units/)
   therefore I feel we should encourage this change, especially if
   we consider that it is important that our data can easily be
   understood by users from other disciplines;

-- about blanks within units: parsing and editing expressions
   containing units becomes really more complex if you allow blanks,
   unless you require non-breakable spaces, but you then require
   non-ascii characters: just being sure that the 'km' and 's-1'
   are not broken in 2 entities on 2 lines becomes non-trivial if
   you allow blanks within units.

See you soon,
Francois

>
>Arnold Rots wrote:
>> Aside from my more impassioned please below, there are two issues that
>> I would like to raise:
>>
>> 1. Why not just refer to the Units section in FITS WCS Paper I?
>
>This discussion of units in FITS is now incorporated into section 4.3 of
>the latest FITS Standard document.
>
>> 2. By outlawing cgs units you are going to alienate the HEA community.
>>    As much as I dislike cgs units (after all, I grew up as a radio
>>    astronomer), I do need to put a word in defending the interests of
>>    HEA - they will be completely lost without their ergs and cm.
>>
>>   - Arnold
>
>I second this comment.  In particular, 'erg' and 'gauss' are perhaps the
>2 most commonly used cgs units that the VO should support. 'Jy' (for
>Jansky) is another non-SI unit that probably should be supported.
>Presumably 'cm' is an acceptable SI unit string, so supporting it is not
>an issue.   Also, SI seems to accept (but not encourage) certain other
>traditional units, including deg, arcmin, arcsec, and angstrom.  Are
>these allowed in the VO services?
>
>Finally, not allowing a space as a legal separator in compound units
>(e.g 'N m' instead of 'N.m') seems overly restrictive.  Is this really a
>problem for some parsers?
>
>-Bill Pence
>
>> Anita M. S. Richards wrote:
>>> Dear Steve,
>>>
>>> Thanks very much for your careful reading of the document.
>>>
>>>
>>>> I may be stealing the words of Arnold Rots when I write
>>>> MJD is not a unit.  It is a measure of time which has
>>>> an origin distinction akin to that of K or degC for temperature.
>>> I apologise if the document is worded clumsily, as I remember Sebastien
>>> making a related point although the sutlety was lost on me.  However, MJD
>>> (like some aspects covered by ISO-8601) is a label for a unit of elapsed
>>> time.  We need to make sure that we can handle common use cases and that
>>> involves knowing the origin of coordinate systems.  Perhaps someone can
>>> come up with a more semantically correct term for unit labels like MJD.
>>
>> NO NO NO!!!
>> MJD (or JD, for that matter) is NOT a unit.
>> It is a measure of time that happens to have the unit 'd'.
>>
>>>> Does the VO effort intend to address the difference between
>>>> meters and seconds in "TT units" and "TCB units"
>>>> and "TCG units"?
>>>> I.e., as seen on page 12 of IERS conventions 2003 (tech note 32)
>>>> a quantity of length or time has a different numerical value "x"
>>>> depending on the reference frame in which its value is expressed
>>>> x_{TDB} = x_{TCB} * (1 - L_B)
>>>> x_{TT}  = x_{TCG} * (1 - L_G)
>>>> where L_G is defined as 6.969290134e-10
>>>> and L_B is approximately 1.55051976772e-8
>>>> and whereas the IAU recommends measurements be in TCG or TCB units,
>>>> a common practice is to express them in TDB/TT units.
>>> I think that if there is a use case and a suggested solution, that could
>>> be included, although the conversion rules are not for us to define (if
>>> possible!), but for us to point to libraries or quote where they are
>>> defined.
>>
>> I think this is a strong argument for specifying a complete (and
>> consistent) AstroCoordSystem in STC: the Time Scale in that element -
>> whether TT, TCG, TDB, or TCB - should resolve this ambiguity.
>> And, if I may add, this argues that a full STC element should be
>> considered the right thing to do for space-time (and related)
>> coordinate, rather than just picking a smattring of individual
>> elements from the STC schema.
>>
>>> Best wishes
>>>
>>> Anita
>>>
>>> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>>>
>>> Dr. A.M.S. Richards, UK ARC Node/AstroGrid,
>>> Jodrell Bank Centre for Astrophysics, Alan Turing Building,
>>> University of Manchester, M13 9PL
>>> +44 (0)161 275 4124
>>> and
>>> MERLIN/VLBI National Facility, Jodrell Bank Observatory,
>>> Cheshire SK11 9DL, U.K. +44 (0)1477 571321 (tel) 571618 (fax)
>>>
>>> "Socialism or barbarism?" Rosa Luxemburg (1915)
>>>
>>> On Tue, 19 May 2009, Steve Allen wrote:
>>>
>>>
>> --------------------------------------------------------------------------
>> Arnold H. Rots                                Chandra X-ray Science Center
>> Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
>> 60 Garden Street, MS 67                              fax:  +1 617 495 7356
>> Cambridge, MA 02138                             arots at head.cfa.harvard.edu
>> USA                                     http://hea-www.harvard.edu/~arots/
>> --------------------------------------------------------------------------
>--
>____________________________________________________________________
>Dr. William Pence                       William.Pence at nasa.gov
>NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
>Greenbelt MD 20771                      +1-301-286-1684 (fax)
>
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Francois Ochsenbein    ------   Observatoire Astronomique de Strasbourg
   11, rue de l'Universite 67000 STRASBOURG  Phone: +33-(0)390 24 24 29
Email: francois at astro.u-strasbg.fr (France)    Fax: +33-(0)390 24 24 17
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