VOunits draft

[William Pence]

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)