IVOA DM Work Package : Units description and usage

!!! Work in progress : Site under re construction !!!

• lead Anita Richards with help from MireilleLouys

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OldUnitsPage

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Goal

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
The main discussion will be initiated at the Interop meeting 2007 27 Sep (Cambridge) and recorded in the DM forum and on this page, but if necessary we will use email between a small group to thrash out details (as for Characterisation).

Existing Documents

• Standards
  • Standards for Astronomical Catalogues, Version 2.0
  • Type `units' in many types of Linux
  • 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.
• Comparison between IAU recommendations, VizieR, HEASARC and the GNU software Units from SebastienDerriere
• Existing VO usage
  • VOSpec model - described in Dimensional Analysis applied to Spectrum Handling in VO context (Osuna & Salgado)
  • Unit strings allowed in STC from ArnoldRots
• More:
  • NIST (National Institute of Standards & Technology) project UnitsXML from MarieLiseDubernet
  • FITS FITS references page, development
    • Section 4 in Greisen and Calabretta 2002, A&A 395, 1061 - see Specification of Physical Units within OGIP FITS files (not only relevant to FITS)

Recommendations

As discussed at IVOA, Trieste, May 2008

• Goal: to provide schemata and 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.

Use cases

• 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.
• TAP - source catalogue DM - need to compare quantities
• 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.

• Cone, S?AP, registry searches
  • The current IVOA standards mandate units, but in some cases this repeatedly leads to problems e.g..
    • Anomalous results are often obtained due 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).
• Database queries: We have no control over the units used by data providers (?). We do need to be able to recognise them.
• Enabling convenient image display with axes labeled in user-specified units/sensible SI multiples.

Interactions with other WG efforts

• IVOA VO Query Language

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IVOA.AnitaRichards - 16 Jun 2008