Vocabulary: http://www.ivoa.net/rdf/datalink/core
Author: François Bonnarel
Date: 2020-06-03
Supercedes: VEP-001

New Term: counterpart
Action: Addition
Label: Counterpart
Description: 
  Data products sharing the target of the experiment or observation
  that led to #this but of unrelated provenance.  This could be
  observations of the same object in different wavelengths or along
  different axes (time, spectrum), but spectra of dust of common
  origin but obtained by different laboratories would be #counterparts 
  as well.

Used-in:
   Various SVO ConeSearch services (courtesy Carlos Rodrigo/SVO), for
   instance
   http://svo2.cab.inta-csic.es/vocats/catlib/dl.php?ID=Coma+T+102&from=cs

   Also, in the GAVO data center, there it is used to link repeated
   observations of a given field among each other.  For instance:
   http://dc.g-vo.org/flare_survey/q/mdl/dlmeta?ID=ivo%3A//org.gavo.dc/%7E%3Fflare_survey/data/plates/ESO040_004362.fits

Rationale:
    Astronomers often want to associate #this (astronomical objects,
    sources or datasets ) with dataproducts of other provenance (but
    sharing some common features with  them). The purpose maybe for
    comparison, cross-corelation, multi-band interpration, follow-up of
    changes, checking simulations results versus observations.

    Examples of that are measurements in another band, images, cubes,
    spectra, timeseries of a source, same dataproduct type and location
    in physical space but at another time, etc.

    In discussions leading up to the VEP, several other terms have been 
    investigated and found inadequate.  #see-also was considered much 
    too general, essentially applying to all datalink items; #followup did
    not match the intended concept, as the linked data products may be
    much older than #this; #cross-associated was considered in more
    detail and might be a contender; #is-observation-of did not quite
    fit because the #counterpart is not an observation of #this but
    something observed by #this.