enhance position bounds to include coverage up to all-sky

[pdowler]

The current Polygon definition is limited to less than all-sky. A MultiPolygon with two hemispheres could be constructed, but you cannot create an outer simple polygon that contains it.

[pdowler]

Whatever we do has to:

• be exposed from TAP via DALI polygon or multipolygon or STC serialisation
• be implementable in database so TAP queries return correctly (already issues lurking here)

[pdowler]

A caom2 MultiPolygon that describes two opposite hemispheres already means all-sky, but the simple DALI polygon in Plane.position.bounds cannot. There is work within the IVOA to augment DALI with support for multipolygon and shape (polymorphism) and this may impact how the CAOM Polygon and MultiPolygon will work

[pdowler]

tentatively added to UML using what was discussed for DALI

[pdowler]

The DALI polygon will not be extended to have a special value for all sky. There will be a polymorphic DALI "shape" that allows polygon or circle values (and this matches the type of Plane.position.bounds).

In CAOM, all sky can be specified (in Plane.position.bounds) as a Circle with radius 180 (degrees). Anything less than that can be specified as a DALI polygon.

note: Limitations in back end implementations will cause these to fail (at least at CADC) until we can implement code to handle these cases and remove limits on maximum polygon size.