SSA_v1_1 < IVOA

---+ SSAP V1.1 Collaborative Page

---
----+ SSAP/SDM Documents

The working draft SSA/SDM V1.1 documents can be found here:

   [to be added]

The current SSA/SDM standards documents can be found here:
   * [[http://www.ivoa.net/Documents/latest/SSA.html][Simple Spectral Access V1.04 (1 Feb 2009)]]
   * [[http://www.ivoa.net/Documents/latest/SpectrumDM.html][Spectrum Data Model (29 Oct 2008)]]
   * [[https://webtest.aoc.nrao.edu/ivoa-dal/ssap-keywords.xls][SSA/SDM model summary spreadsheet]]


---

---+ SSAP/SDM Document Update Discussion

At the <nop>EuroVO AIDA a small group of people interested in updating the
SSA and <nop>SpectrumDM document gathered. Participants; IVOA.BrunoRino, IVOA.KeithNoddle (DAL chair), IVOA.MireilleLouys (DM chair), IVOA.AlbertoMicol, IVOA.IgorChilingarian, IVOA.JesusSalgado, IVOA.FrancoisBonnarel.

Separately from this meeting, from the US side, IVOA.DougTody and
IVOA.JonathanMcDowell (the SSAP and SDM document editors) had also been
discussing these proposed changes with the document authors and others
via email.

We set forth the following goal: To create 1.1 versions (of SSA and
the <nop>SpectrumDM) that attempt to fix or clarify what were seen as
inconsistencies between the two documents.
Mor extensive clarifications or additions could be added in
a version 1.2. Big changes, possibly not backwards compatible, are
postponed to 2.0 versions.

The rationale for this split is to have version 1.1 approved quickly
and replace the current 1.0. A version 1.2 would permit more changes
but would take significantly longer, gathering more extensive 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.

----+ SSAP/SDM Inconsistencies
IVOA.BrunoRino (26/03/10) analysis:

1. *The SSA data model is derived, but decoupled, from the <nop>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 <nop>SpectrumDM
- the <nop>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 <nop>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 <nop>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 <nop>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 "*":

<nop>Char.SpatialAxis.SamplingPrecision.SampleExtent
<nop>Char.SpatialAxis.SamplingPrecision.FillFactor
<nop>Char.SpectralAxis.SamplingPrecision.SampleExtent
<nop>Char.SpectralAxis.SamplingPrecision.FillFactor
<nop>Char.TimeAxis.SamplingPrecision.SampleExtent
<nop>Char.TimeAxis.SamplingPrecision.FillFactor

4. *Missing UCDs in SSA*
The SSA is correct, the <nop>SpectrumDM should not have a UCD for the
following elements (the UCDs provided in the <nop>SpectrumDM on those
elements are either wrong or confusing):

*utype*                 *UCD to be removed*
<nop>Dataset.TimeSI          time;arith.zp
<nop>Char.SpatialAxis.Name   meta.id
<nop>Char.SpatialAxis.Ucd    meta.ucd
<nop>Char.SpatialAxis.Unit   meta.unit


5 *Misc. typos*
The <nop>SpectrumDM (on the FITS serialization section) should fix the
following utypes:

<nop>Spectrum.Curation.ContactName  ->  Spectrum.Curation.Contact.Name
<nop>Spectrum.Curation.ContactEmail  ->  Spectrum.Curation.Contact.Email
<nop>Spectrum.Char.SpatialAxis.Accuracy.StatErr  ->
<nop>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 <nop>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



7. *Wrong UCDs*

<nop>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



8. *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)



9. *Inconsistencies within the <nop>SpectrumDM itself*

Add a comment to the FITS serialisation stating that it does not cover
the whole of the <nop>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.

See also:
http://www.ivoa.net/internal/IVOA/IvoaExecMeetingFM34/SSA_SDM_assessment.xls

----+ SSA and SDM Documents Update (response to above)
IVOA.DougTody (01/04/10) analysis:

I am trying to synthesize these various comments on SSA/SDM.  I think
I mostly agree with the detailed changes suggested.  The comments here
are mostly higher level.

At this point it appears that both Jonathan and Bruno agree that the
SSA metadata and the SDM, while quite similar, are not quite the same
thing (hence where there are differences they are not necessarily
"inconsistencies" but are restrictions in units, mandatory fields
etc. required by the specific application).  SSA defines the data
model used in the query response, describing the data which a
service can deliver.  SDM defines the data model of a spectrum
dataset/file/instance.

A key point which has not been adequately discussed is the role of
the generic GDS/Obs data model.  While the SDM explicitly describes
a Spectrum instance, most of what is in the SSA query response is
actually <nop>GDS/ObsDM metadata and not specific to Spectra at all - at
this level the metadata should be the same for an image, for an SED,
for a time series, etc.  It does not make a whole lot of sense for
example to have Spectrum.Target.Name and Image.Target.Name and allow
these to be two different things.  An application which deals with
multiple types of data would like global and generic metadata such
as this, which is applicable to any type of data, to be usable as
just "Target.Name" (possibly in a future version something such as
Obs.Target.Name would be preferable, but we did not have that when
SSA was defined).  Once we have a full set of DAL2 services one would
like to have the common stuff factored out so that it does not have
to be customized for every type of data.  For the purposes of data
discovery and description most of this stuff is the same.

The above is the reason why the "Spectrum" prefix does not carry over
into the SSA query response.  Rather, essentially all of the SSA query
response metadata is either generic dataset metadata, or specific to
the service mechanism itself and hence also generic and applicable to
all such services.  It is not actually from the SDM at all, we merely
require them to be consistent.  That small part of the query response
metadata which is not generic and which is specific to the type of
dataset being queried (Spectrum, Image, whatever) is what is in the
"Dataset" component model.

In other words, for a DAL2 query response what we have is virtually
all generic dataset / Obs metadata, plus a separate "Dataset" element
which is specific to the type of dataset being queried.  This generic
query response is supposed to be the same for all of the "typed"
services derived from the generic Dataset.  The data model (if any is
formally defined) for a specific type of dataset such as Spectrum,
Image, etc. also inherits the generic dataset metadata.  Ideally we
want to have all these be internally consistent for a particular major
version of the interface (1.x, 2.x, etc.).  Once we have SSA, SIAV2,
<nop>ObsTAP, etc. it is clear that it could be attractive to standardize
the generic metadata - which comprises most of the metadata returned -
among all these very similar services.

Now there are some things which could possibly be done differently
several years later, for example the "Dataset" component in the query
response might instead take on the class name of the specific type
of data which it describes, such as Spectrum or Image.  Either approach
is possible and it doesn't make all that much difference.

The full discussion from which the above was taken is available at:
http://www.ivoa.net/forum/dal/1004/1690.htm



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Attachments
Topic attachments
I	Attachment	History	Action	Size	Date	Who	Comment
xls	ssap-keywords-rc2.xls	r1	manage	102.0 K	2010-05-14 - 04:17	DougTody	SSA V1.1 data model spreadsheet
doc	wd-ssa-1.1-20100513.doc	r1	manage	784.0 K	2010-05-14 - 04:16	DougTody	SSA V1.1 working draft (13 May)(doc)
pdf	wd-ssa-1.1-20100513.pdf	r1	manage	1200.2 K	2010-05-14 - 04:15	DougTody	SSA V1.1 working draft with markup (13 May)(pdf)
This topic: IVOA > SSA_v1_1
Topic revision: r6 - 2010-05-14 - DougTody