SSA-1_1-Err-2 < IVOA

---+ SSA v1.1 Erratum 2: Invalid UCDs required in specification 1.0

Author: Mireille Louys

Date last changed: 2022-01-17

Date accepted: 2022-03-08

Github Issue: <a target="_top" href="https://github.com/ivoa-std/SSA/issues/1">SSA Issue #1</a>

---++ <a name="Rationale"></a>Rationale

Appendix D: SSA Data Model Summary, PDF page 64, lists the data model elements supported by the SSA specification with Utypes and UCDs.

Some of the UCDs proposed lead to ambiguous interpretation and do not follow the combination rules of the UCD specification. Specifically, __instr.fov__ must be secondary. Hence, the UCDs for the fields with utypes

   * !Char.SpatialAxis.Coverage.Bounds.Extent
   * !Char.SpatialAxis.Coverage.Support.Area, and
   * !Char.SpatialAxis.Coverage.Support.Extent
as prescribed by SSAP 1.1 are invalid and need to be repaired.

This erratum provides valid UCDs, which not only have proper semantics but also enable UCD-aware clients to determine the nature of the fields if they do not know the SSAP data model.

---++ <a name="Erratum_Content"></a>Erratum Content

This erratum makes the following changes in the table P. 64:
| Utype | UCD | Definition | data type |
| !Char.SpatialAxis.Coverage.Bounds.Extent | *phys.angSize;instr.fov* | Aperture angular size | double |
| !Char.SpatialAxis.Coverage.Support.Area  | *pos.outline;instr.fov* | Aperture region | char * |
| !Char.SpatialAxis.Coverage.Support.Extent  | *phys.angArea;instr.fov* | Field of view area, sq. deg. | double |

---++ <a name="Impact_Assessment"></a>Impact Assessment

SSAP clients do not identify fields by UCDs and hence are unaffected by this change.

Clients relying on UCDs to determine how to treat individual columns will now be able to gain some basic understanding of the affected columns, which they could not before (as the UCDs were invalid and hence meaningless).

Most importantly, valid SSAP services will raise fewer errors in UCD validation.

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Topic revision: r5 - 2022-03-09 - MarcoMolinaro