Characterisation draft

[louys at alinda.u-strasbg.fr]

Dear Anita,

> Mireille:
>>
Sorry if I was unclear here:
I just wanted the things concerning Axis to be described first, and 
then Coverage, because it is the order we can illustrate in the XML 
Schema.

Something like this:

AXES

  A description should provide at least one axis ("AxisFrame", "Axis").

  The unit and coordsystem 'must' be given for each Axis present (these
  may be relative to an internal reference only, e.g. (x,y) spatial
  coordinates. In such a case the Location and Bounds 'must' be given on
  that axis).

  All three of the Space, Time and Spectral Coverage Axes 'should' be
  given\footnote{some might be considered irrelevant for simulated data, or
  not conventionally provided e.g. for old spectra with no time stamp}.

  The Observable Axis 'should' be given. \footnote{its omission may seem
  reasonable if publishing e.g. the coverage intended for a future
  survey}.
  In this case, the Axis can be defined , but the Coverage could be 
empty, or        rather undefined or unknown ???

  Other Axes e.g. Velocity 'may' be given, with their Coverage.


COVERAGE

  The value 'must' be given for either the Location or the Bounds on
  each Axis.
  Values 'should' be given for both Location and Bounds.

  If Location is not given then for some Axes the default Location can
  be the mid-point of Bounds\footnote{in some cases this might be
  complicated (e.g. some spatial coordinates) or impossible}.

  If Bounds are not given then in some cases defaults are possible
  e.g. if a spatial axis has Coordsys ICRF the default would be all
  sky \footnote{ a more restricted coverage might be derived once there
  is a link to Observation and the telescope location.}

  Support 'should' be given, otherwise it defaults to Bounds, if present.


thanks, Mireille