SSA and SDM inconsistencies: the AIDA meeting minutes

[Bruno Rino]

Hello all,

At the EuroVO AIDA a small group of people interested in updating the
SSA and SpectrumDM document gathered. These are the minutes.

We set forth the following goal: To create 1.1 versions (of SSA and
SpectrumDM) that attempt to fix the inconsistencies between the two
documents. Clarifications and small additions should be added in
versions 1.2. Big changes, possibly not backwards compatible, are
postponed to 2.0 versions.


The rationale for this split is to have versions 1.1 approved quickly
and replace the current 1.0. Versions 1.2 will take significantly
longer, gathering much input from the community.

In both cases (1.1. and 1.2) the assumption is that no existing
application should break. This means that when a fix creates potential
breakage, the potentially affected applications should be consulted.

Below, the result of our discussion on what must be changed in order to
reach version 1.1. We were lucky enough to have both the DM and DAL
Working Group leads; we concluded that after a short period of time
after circulating these meeting minutes to the relevant lists, Working
Drafts should be produced, ahead of the May interop in Victoria.

Participants:
Bruno Rino
Keith Noddle
Mireille Louys
Alberto Micol
Igor Chilingarian
Jesus Salgado
François Bonnarel


1. The SSA data model is derived, but decoupled, from the SpectrumDM.

The acknowledged divergences are:
- "required" flags (Mandatory, Recommended, Optional) are different
- the SSA data model contains service related metadata, that have no
meaning for the SpectrumDM
- the SpectrumDM contains metadata related to data analysis (Data.*)
that are of no interest for data discovery (which is the purpose of SSA)
- in SSA, the "Spectrum." prefix found in the SpectrumDM utypes was
dropped, and in some cases a "Dataset." prefix was added.

Even if those divergences are discrepancies, they are not going to be
fixed in a 1.x document. Instead, SSA Section 2.2 ("Data Model") must be
changed to reflect these divergences. It should state explicitly that a
reader of the SSA should only refer to the SpectrumDM to seek
clarification about the meaning of metadata fields. The specification of
required fields, the UCDs, and even the utype syntax for setting up a
SSA server are to be read from the SSA document. Metadata defined in the
SpectrumDM, but not listed in the SSA, are not relevant for SSA service
interface.

This is a compromise towards reaching rapidly a stable revision of the
documents. We would much prefer to have a single source for the
definition of the datamodel, which the SSA protocol would just extend.
But we believe this is too large of a task to achieve while maintaining
backwards compatibility, on a reasonable time-scale.



2. The use of "*" and ".." in UCDs

This must be eliminated.

These characters are always used in the context of "em.*" or "em...".
Our understanding is that these characters are placeholders, which a
data provider must fill in, according to its requirements. A list of all
possible values and meanings must be provided instead, using the
following primary UCDs:
em.wl
em.freq
em.energy



3. The use of "*" in utypes

This must be eliminated. The correct utypes to use are the ones in the
"Query Response" section of SSA, but without the "*":
Char.SpatialAxis.SamplingPrecision.SampleExtent
Char.SpatialAxis.SamplingPrecision.FillFactor
Char.SpectralAxis.SamplingPrecision.SampleExtent
Char.SpectralAxis.SamplingPrecision.FillFactor
Char.TimeAxis.SamplingPrecision.SampleExtent
Char.TimeAxis.SamplingPrecision.FillFactor


4. Missing UCDs in SSA

The SSA is correct, the SpectrumDM should not have a UCD for the
following elements (the UCDs provided in the SpectrumDM on those
elements are either wrong or confusing):
*utype*                 *UCD to be removed*
Dataset.TimeSI          time;arith.zp
Char.SpatialAxis.Name   meta.id
Char.SpatialAxis.Ucd    meta.ucd
Char.SpatialAxis.Unit   meta.unit


5. Misc. typos

The SpectrumDM (on the FITS serialization section) should fix the
following utypes:
Spectrum.Curation.ContactName  ->  Spectrum.Curation.Contact.Name
Spectrum.Curation.ContactEmail  ->  Spectrum.Curation.Contact.Email
Spectrum.Char.SpatialAxis.Accuracy.StatErr  ->
Spectrum.Char.SpatialAxis.Accuracy.StatError

The SSA should fix the following UCD:
em;spec.binSize  ->   em;spect.binSize

Extra spaces in UCDs and utypes are typos and should be removed


6. Dimensional analysis typo:

In the SpectrumDM, change (from 10-10 to 1E-10) the way to express
exponents within the dimensional analysis elements

Section 3.2 should read:

Pedro Osuna and Jesus Salgado have proposed a representation in the
spirit of dimensional analysis, using the symbols M, L, T to signify kg,
m, s respectively and omitting the ** for powers, so that
10**3 Jy Hz
which is equivalent to
10**-23 kg s**-2
is written compactly as
1E-23MT-2

and the example in section 9.4:

SPECSDIM= '1E-10 L'             / Spectral SIDim
FLUXSDIM= '1E+7 ML-1T-3'       / Flux SDim



6. Wrong UCDs

Spectrum.Char.SpectralAxis.Coverage.Location.Value has a wrong UCD of
instr.bandpass.
It should become the following list (in accordance to point 2 above):
em.wl;instr.bandpass
em.freq;instr.bandpass
em.energy;instr.bandpass



7. Inconsistencies within the SSA itself

Add the Dataset.Deleted utype to Appendix D.

Remove the Data.* utypes from Appendix D.

Add the remaining missing utypes present in Appendix D to section 4.2
(the list is too long and too boring to show here)



8. Inconsistencies within the SpectrumDM itself

Add a comment to the FITS serialisation stating that it does not cover
the whole of the SpectrumDM utypes


Note:
The "consolidation" activities detailed in points 7 and 8 should also
make sure the order by which the utypes are listed is consistent
throughout all documents.