IVOA DM Work Package : Units description and usage

!!! Work in progress : !!!

  • lead Anita Richards
  • units final VO standard to be achieved with the Semantic WG


DiscussionOnVOUnits : Discussion on specific aspects of the document 1.0.

WD-VOUnits-1.0-20111216.pdf 2011 December 16 - first release of Version 1.0

WD-VOUnits-v0.3-20110508.pdf 2011 May 08 draft IVOA Note minor update

WD-VOUnits-v0.2-20090522.pdf 2009 May 22 draft IVOA Note

2009 March Draft IVOA working draft

ImplicationsForADQL 2009Mar18


Main IvoaDataModel page


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.

Existing Documents


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: 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?

Use cases

  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.
    Reference: SPECFIND Catalog of radio continuum spectra
    Authors: Vollmer, B.; Davoust, E.; Dubois, P.; Genova, F.; Ochsenbein, F.; van Driel, W.
    Publication: VizieR On-line Data Catalog: VIII/74.
    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.
      SELECT a.fint/(a.maj*a."min") 
      as fluxdensity from firstSource as a
      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 (a.maj*a.min/4*ln2)
      • SELECT 365.25*a.dist_opt/(a.jdate-2450975.0) 
        as propermotion FROM twomass_psc as a
        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.

Interactions with other WG efforts

IVOA.AnitaRichards - 16 Jun 2008


Topic attachments
I Attachment History Action Size Date Who Comment
Texttxt STCunits.txt r1 manage 5.9 K 2007-10-05 - 16:52 ArnoldRots Unit strings allowed in STC
PDFpdf WD-VOUnits-1.0-20111216.pdf r1 manage 526.2 K 2011-12-16 - 23:15 SebastienDerriere VOUnits WD 1.0 2011-12-16
PDFpdf WD-VOUnits-v0.3-20110508.pdf r3 r2 r1 manage 385.6 K 2011-05-09 - 08:17 MireilleLouys WD after interop May 2009/ minor update
Texttxt minutes_units_discussion_070927.txt r1 manage 1.3 K 2007-10-01 - 06:21 AlbertoMicol Minutes of initial discussion: Cambridge, 20070927
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Topic revision: r41 - 2011-12-16 - SebastienDerriere
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