(r28) UnitsDesc < IVOA

IVOA Web>IvoaDataModel>UnitsDesc (revision 28) (raw view)~~EditAttach~~

<H1>IVOA DM Work Package : Units description and usage</H1>
<br/>
 %RED% !!! Work in progress : Site under re construction !!! %ENDCOLOR%                

   * lead [[IVOA.AnitaRichards][Anita Richards]] 
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[[%ATTACHURL%/Units.html][Draft IVOA working draft]]

OldUnitsPage

[[IVOA.IvoaDataModel][Main IvoaDataModel page]]
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<H2>Goal </H2>
This workpage is meant to record the general practice in manipulating units for astronomical data and define a simple way to homogeneise their representation within VO services.



<H2> Existing Documents </H2>
   * Standards
      * [[http://vizier.u-strasbg.fr/doc/catstd-3.2.htx][Standards for Astronomical Catalogues, Version 2.0]]
      * Type `units' in many types of Linux
      * [[http://www.iau.org/Units.234.0.html][IAU Recommendations concerning Units from the IAU Style Manual by G.A. Wilkins (1989)]]
         * Symbols unsuitable for the IVOA as they  use Greek letters, diacritical marks etc. which are not easy to use in machine-readable documents; escaped versions could be error-prone. These are not in general used within data (as distinct from free text) anyway.
   * [[IAUVizieRHEASARCGNU][Comparison between IAU recommendations, <nop>VizieR</nop>, HEASARC and the GNU software Units]] from IVOA.SebastienDerriere
   * Existing VO usage
      * VOSpec model - described in [[http://arxiv.org/pdf/astro-ph/0511616][Dimensional Analysis applied to Spectrum Handling in VO context]] (Osuna & Salgado)
      * [[http://hea-www.cfa.harvard.edu/~arots/nvometa/STC/STCunits.txt][Unit strings allowed in STC]] from IVOA.ArnoldRots
   * More:
      * [[http://www.mel.nist.gov/msid/sima/07_ndml.htm][NIST (National Institute of Standards & Technology) project UnitsXML]] from IVOA.MarieLiseDubernet 
      * *FITS* [[http://fits.gsfc.nasa.gov/fits_wcs.html][FITS references page]], [[http://fits.gsfc.nasa.gov/fits_draft.html][development]]
         * Section 4 in Greisen and Calabretta 2002, A&A 395, 1061 - see [[http://heasarc.gsfc.nasa.gov/docs/heasarc/ofwg/docs/general/ogip_93_001/ogip_93_001.html][Specification of Physical Units within OGIP FITS files]] (not only relevant to FITS)

<H2>Recommendations</H2>

As discussed at IVOA, Trieste, May 2008   

   * Goal: to provide suitable tools - Backus Naur form?, libraries and references to allow 
      * Recognition of units in use by the members of IVOA
      * Simple (mostly linear) conversions such as metres/parsec; use of SI prefixes (k,M,G eetc.) to avoid bulky numbers or damaged precision.  
   * Whatever is adopted should as far as possible be compatible with existing VO implimentatations and FITS, and should cover all the relevant IAU types of unit (as distinct from the unsuitable symbols). 
      * *Please nominate whichever you consider to be the most complete existing system*
     The IVOA should provide translation where there are unambiguous variations in other systems, including from FITS keywords (making use of existing tools/libraries where possible).
   * *Please identify any serious ambiguities* e.g. k could be kilo- or Boltzmann's constant, and suggestions for how to overcome this (recognise by context? Flag for human intervention?).
   * Produce web page of links to existing tools for unit handling, conversion, dimensional analysis etc.
      * Example from CDS pages: [[http://cdsweb.u-strasbg.fr/devcorner.gml][see Unit conversion link]] - but in a form which can be linked to user programs or packages in C, IDL, python etc. with simple instructions
   * Educate next generation of data providers to use _some_ standard for units!
   * Need to record date and reference to values of fundamental quantities where available.
   * *How far can we go with dimensional analysis?* Are there so few cases where it breaks down that these can be handled individually, and if so how does code recognise them (e.g. the Hubble constant, in km/s per Mpc) - or can we specify limited situations where dimensional analysis is adequate?

<H2>Use cases </H2>
   1. TAP - source catalogue DM - need to compare quantities _please elaborate_
   2. SPECFIND : A tool to extract cross-identifications and radio continuum spectra from radio catalogues contained in the VIZIER database of the CDS. <BR> Reference:   SPECFIND Catalog of radio continuum spectra <BR> Authors:     Vollmer, B.; Davoust, E.; Dubois, P.; Genova, F.; Ochsenbein, F.; van Driel, W. <BR> Publication: <nop>VizieR On-line Data Catalog: VIII/74. <BR> _please elaborate_ - is this a requirequirement for conversion between physical flux units e.g. mJy, Jy, W m-2 Hz-1 - what about Jy beam-1 (if you are lucky the beam size is given conveniently...?
   3. Cone, S?AP, registry searches
      * The current IVOA standards mandate units, but this leads to frequent problems due to to a Cone search/SIAP service returning a region in different units from the one the user thinks they are using (e.g. 0.1 arcmin instead of degree) (see DB queries below).
   4. Database queries: We have no control over the units used by data providers (?).  We do need to be able to recognise them.
      * Example inspired by Jeff Lusted: What can ADQL do about units for synthetic columns? E.g. <pre>SELECT a.fint/(a.maj*a."min") <br/> as fluxdensity from firstSource as a</pre> where fint has units of mJy and maj and min have units of arcsec so fluxdensity has units of mJy arcsec-2
      * More complicated cases:  
         * In the above case, strictly speaking the divisor should be <tt>(a.maj*a.min/4*ln2)</tt>
         * <pre>SELECT 365.25*a.dist_opt/(a.jdate-2450975.0) <br/> as propermotion FROM twomass_psc as a</pre> where the inputs have units of arcsec and Julian Days (implicit for 2450975.0) but the user wants output in arcsec per year ...
   5. Enabling convenient image display with axes labeled in user-specified units/sensible SI multiples.
   6. Complex conversions e.g. magnitudes to physical units, are outside our scope but well-defined physical units should make that workpackage easier. 
      * Flux unit conversions e.g.  VOSpec, tools using the STILTS library
      * Coordinate conversions (B1950 RA-Dec to J2000 glat-glon etc.)  e.g. tools using the STILTS library, SIMBAD services etc.
      * Conversion between wavelength and frequency/energy units is non-linear (even without observational issues).  Whilst this is trivial (and rounding errors are unimportant) for single simple regions, it makes it hard to describe a spectral resolution of 1 MHz between 1 and 30 GHz, with a single number in wavlength units.

<H2>Interactions with other WG efforts</H2>
   * [[IvoaVOQL][IVOA VO Query Language]] 

---IVOA.AnitaRichards - 16 Jun 2008
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Attachments

Topic attachments
I	Attachment	History	Action	Size	Date	Who	Comment
txt	STCunits.txt	r1	manage	5.9 K	2007-10-05 - 16:52	ArnoldRots	Unit strings allowed in STC
txt	minutes_units_discussion_070927.txt	r1	manage	1.3 K	2007-10-01 - 06:21	AlbertoMicol	Minutes of initial discussion: Cambridge, 20070927
gif	units2.gif	r1	manage	14.8 K	2008-10-17 - 17:47	AnitaRichards

Topic revision: r28 - 2008-10-17 - AnitaRichards

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