STC2:Coords Proposed Recommendation: Request for Comments #2WATCHOUT: This RFC page has been replaces RFC#1 Rationale for a second RFC round:
<a name="Why RFC #2"></a> SummaryTBC Many comments were collected after RFC #1. Some of them just required text împrovements but some others implied significant model changes, especially for Coords. A global RFC answer has been sent to WG mailing list in February 2020: answer. Theses models do not aim at being used as such to describe dataset classes or DAL responses. They must be seen as component boxes for top level models such as CubeDM or Mango. This is the reason why Measurements and Coords have been split. Measurements cannot be used without Coords because the lastest in imported by Measurements but Coords can be used in some other contexts e.g. Transform. The nature of the changes in Coords, that impacts Measurements as well, led the TCG to decided on 2020-10-8 a second RFC round for both model.SummaryVersion 1 of STC was developed in 2007, prior to the development and adoption of vo-dml modeling practices. As we progress to the development of vo-dml compliant component models, it is necessary to revisit those models which define core content. Additionally, the scope of the STC-1.0 model is very broad, making a complete implementation and development of validators, very difficult. As such it may be prudent to break the content of STC-1.0 into component models itself, which as a group, cover the scope of the original. This effort will start from first principles with respect to defining a specific project use-case, from which requirements will be drawn, satisfied by the model, and implemented in the use-case. We will make use of the original model to ensure that the coverage of concepts is complete and that the models will be compatible. However, the form and structure may be quite different. This model will use vo-dml modeling practices, and model elements may be structured differently to more efficiently represent the concepts. This model describes the Coordinates model and covers the following concepts.
Implementation Requirements(from DM Working group twiki): The "IVOA Document Standards" standard has a broad outline of the implementation requirements for IVOA standards. These requirements fit best into the higher level standards for applications and protocols than for data models themselves. At the Oct 2017 interop in Trieste, the following implementation requirements for Data Model Standards was agreed upon, which allow the models to be vetted against their requirements and use cases, without needing full science use cases to be implemented.
VO-DML Validation:
Serializations:
Software:
ValidatorsAs noted above, the serializations may be validated to various degrees using the corresponding schema
Links with MeasThe Coordinates model a base component, primarily used to support the development of other models. Most significantly is in support of the Measurement model, which is the core model of interest to most use cases. Information about the relation to that model, how the use case requirements divide up, etc. can be found on the STC2 pageComments from the IVOA Community during RFC/TCG review period: 2020-10-26 - 2020-12-07The comments from the TCG members during the RFC/TCG review should be included in the next section. In order to add a comment to the document, please edit this page and add your comment to the list below in the format used for the example (include your Wiki Name so that authors can contact you for further information). When the author(s) of the document have considered the comment, they will provide a response after the comment. Additional discussion about any of the comments or responses can be conducted on the WG mailing list. However, please be sure to enter your initial comments here for full consideration in any future revisions of this document | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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> > | Comments by Markus Demleitner(1) I'm really unhappy with what the document gives as "use cases". To me, a use case says something like "User is doing X, client can do Y because of what we're specifying here". Only from something like that one can reliably derive requirements -- and avoid discussions like "should features A and B be in there at this moment?" Contrast this with "exercise the transform model" -- I can derive anything or nothing from that. Can we have use cases that actually describe uses, as in "Bring two catalogues on the same epoch" or "Overplot objects from a catalogue on a calibrated image"? (2) As in the STC1 days, I still disagree quite strongly with treating JD, MJD, and ISOTime at the modelling level. It's true that ISO time is a bit special in that it's not just a float continuing on and you certainly don't want free offsets on that, but still: that's container-level serialisation, and JD and MJD aren't special in any way over any other sort of floating-point time. Can't we just follow what TIMESYS in VOTable does? (3) The cardinal sin of any data model is trying to do too much; since polarisation sticks out as not really related to anything in this model and it's only mentioned in a single requirement the source of which remains unclear: Can't we take it out and save it for later, when we have real use cases? (4) What would the ~coordSys of a BinnedCoordinate be? Talking about BinnedCoordinate: I don't think it's a good idea to talk about pixels too much here -- most people will encounter pixels as a coordinate system on their CCD frames, and there, they normally don't look at integral pixels, and they very certainly shouldn't use BinnedCoordinates. Come to think of it: When would they use them anyway? Perhaps that's another thing we can postpone? (5) Since all times are offsets (from what's called their epoch in timekeeping), I fail to see why we would have TimeOffset and TimeInstant as separate classes. (6) Since we know about TimeInstant: Why would Epoch not just use that but rather uses an ad-hoc time serialisation? (7) I still think it's a bad idea to consider temperature or flux as "coordinates". If you want "coordinate" to mean anything, it's linked to vector spaces, and neither temperature nor flux make sense as a vector component as such. Why would you want to do that? (8) What's the use case for your Axis objects? For instance, is it expected that RA is explicitly marked as a cyclic axis? I see requirements coords.001 and coords.002 referenced, but I, really, still can't tell what a client would do with this information. (9) I'm afraid I don't find any of the "reference implementations" terribly convincing. There's far too much "work in progress" or "previous version". At least having one thing where we could see as many of the (IMHO too many) features of this model at work as possible would help mollify my concerns that very little of what is written in the spec has actually been tried out. -- MarkusDemleitner - 2020-11-25 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Comments from TCG member during the RFC/TCG Review Period: 2020-10-26 - 2020-12-07WG chairs or vice chairs must read the Document, provide comments if any (including on topics not directly linked to the Group matters) or indicate that they have no comment. IG chairs or vice chairs are also encouraged to do the same, althought their inputs are not compulsory.TCG Chair & Vice ChairApplications Working GroupData Access Layer Working GroupData Model Working GroupAfter having tested the model on different Vizier tables (see the globals section here). I suggest a modification in the sky coordinates .Sky coordinates are now reprensented by a 3D vector (coords:Point). The role of the 3 axis (lon/lat or xyz) is given by the axis descriptions that is part of the coordinate system (coords:SpaceSys). It would be quite more simpler to have an abstract class (e.g. Point) with 4 concrete sub classes:
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> > |
Two spots in the current PR are spots of trouble for Semantics:
(1) You shouldn't include the vocabularies (as in appendix C) in the specification -- that'll only tempt people to use exactly what's mentioned there in their software. A brief comment to the effect that software is encouraged to regularly update their lists or so would of course be welcome, as would be comments on the general architecture or quirks (like the old VOTable COOSYS names). The complete list, however, will just be outdated after a while, and people will curse us because we apparently publish contradicting information. Plus, it'll shave off a few more pages, which might lessen peoples' aversion against takeup.
(2) The vocabulary http://www.ivoa.net/rdf/refframe definitely isn't good for REC yet. In particular:
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Data Curation & Preservation Interest GroupEducation Interest GroupKnowledge Discovery Interest GroupSolar System Interest GroupTheory Interest GroupTime Domain Interest GroupOperationsStandards and Processes CommitteeTCG Vote : Vote_start_date - Vote_end_dateIf you have minor comments (typos) on the last version of the document please indicate it in the Comments column of the table and post them in the TCG comments section above with the date.
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