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    Select observations according to their absolute position. A0 and D0  are
    sexagesimal strings (degrees, or hour angle for A0 in case of equatorial
    coordinates). The coordinate system (and its equinox for equatorial sys-
    tem) must be provided as third (and fourth) argument to the option.

    In practice, the option converts (if they are different) the given coor-
    dinates to the system used by the observation currently considered (e.g.
    galactic  to  equatorial 2000). Then it translates the absolute position
    to relative position. Finally, it compares the offset  position  of  the
    observation  to  this computed reference. SET MATCH is used as tolerance
    (same as FIND /OFFSET). These operations are repeated for  all  observa-
    tions in the file.

    Beware  this  option  is less efficient than its equivalent FIND /OFFSET
    (which uses only index elements). When considering absolute coordinates,
    the  whole  position  section must be read for each observations. As the
    Class Data Format does not enforce consistency of  the  reference  posi-
    tions  of all the observations, all the conversions described above must
    be repeated for all observations. But if you know your file  is  consis-
    tent,  FIND /OFFSET will always be faster for the same result. For exam-
    ple, prefer FIND /OFFSET 0 0 instead of FIND  /POSITION  to  select  the
    spectra near the reference position.

Gildas manager 2019-02-18