[Observation] relation to Dataset

[CresitelloDittmar, Mark]

All,
  This thread is for discussion on the relation between Observation and
Dataset.

ref: ObsCoreDM - http://www.ivoa.net/documents/ObsCore/20111028/index.html
ref: diagram illustrating relation of Image/Spectral Observation to
ObsCoreDM (draft)

http://www.ivoa.net/pipermail/dm/attachments/20131113/c9ef7581/attachment-0001.png

motivation
  It is clear that there is a relationship between "Observation" and a more
generic "Dataset".  This "Dataset" would contain elements such as the
dataProductType, and dataProductSubtype, presumably others.  This object
has not been formally defined.

  In ObsCore, there is an implied relationship for Observation as an
Extension of Dataset in the location of these attributes.  So, I have
always interpreted that Observation "is" a Dataset.  This is reflected in
my choice of the name "ObservationDataset" in the left hand package of my
diagram.  It implies that it is a Dataset extended for Observation purposes.

  Recent discussion brings this relationship into question, with assertions
that an Observation can be associated with 0 or more Datasets.

  This has real ramifications for the Image and Spectral models..

Seed:

If the relation is Observation "has" 0..* Dataset, then all the diagrams to
date are wrong.
It feels like this would be a fundamental change to all these models.

  - there would need to be a bi-directional relation between Observation
and Dataset
       (observation has 0..* Dataset; Dataset associated with 1 Observation)
    Hmm.. since there can be Datasets not associated with Observations,
this would
    need to be a specialization of Dataset.. (ObservationDataset.. but not
the one in my diag.)

  - the Char associated with Observation would characterize the total space
of all included Datasets.  (0..1) relation to Observation.  If no Datasets,
no Char

  - each Dataset would require it's own Characterisation, specific to it's
space.
    (so there is another attribute for Dataset).

  - we would need to specify which of the elements are associated to the
Dataset, and which to the Observation.  e.g. DataModel => Dataset;  Target
=> Observation

Thoughts?
Mark
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