STC in VOTable"> Embedding STC in VOTableAt the Garching Interop, Markus has uttered some gripes with version 1.1-20090612 of the STC in VOTable Note specifying how to embed STC in VOTables. | ||||||||
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< < | There's been quite a bit of work going on since then; more ancient items have been moved to STCInVOTableArchive. | |||||||
> > | There's been quite a bit of work going on since then; older items have been | |||||||
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> > | moved to STCInVOTableArchive, which doesn't necessarily mean the issues are settled; if something discussed there still bugs you, feel free to resurrect it here. | |||||||
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> > | Draft of 2010-04-20 | |||||||
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< < | Change 1: Reverse References | |||||||
> > | I've put up a new draft at | |||||||
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> > | http://vo.ari.uni-heidelberg.de/docs/note_stc_20100420.pdf. This is basically a rewrite (while, I believe, maintaining the spirit of the old note). Below, I've opened up Level 3 discussion spaces for what I believe a the most sweeping changes against the ''previous draft'' (not the current version of the note; see STCInVOTableArchive for the other changes). | |||||||
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< < | Instead of having utype and ref on FIELD, put groups into the AstroCoords group: | |||||||
> > | -- MarkusDemleitner - 20 Apr 2010 | |||||||
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> > | Change: One group | |||||||
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< < | <GROUP ID="lltoush_coo" ref="lltoush" utype="stc:AstroCoords"> <GROUP ref="alpha" utype="stc:AstroCoords.Position2D.Value2.C1" /> <GROUP ref="rv" utype="stc:AstroCoords.Redshift.Value" /> | |||||||
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< < | (or use FIELDrefs that, I'm told, can now take utypes as well). | |||||||
> > | MD: Rather than having one AstroCoordSystem and one AstroCoord group per | |||||||
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> > | system, all utypes dealing with one coordinate system is now contained in one group of (typically) utype stc:ObservationLocation. While this means you can no longer re-use coordinate system definitions within a VOTable, I believe the advantages in implementation simplicity far outweigh this drawback. | |||||||
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< < | Rationale: | |||||||
> > | Change: AbsoluteTime substitution group gets yet another special treatment | |||||||
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> > | MD: STC-X unfortunately has the AbsoluteTime substituion group that gives what in VOTable is xtype's job (and even if that weren't so, having serialization details in utypes is a pain anyway). So, a new special rule | |||||||
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> > | was inserted into the utype generation algorithm to divert ISOTime, JDTime and MJDTime into some special utype, and there's a recommendation to drop that particular utype. Nasty special-casing all over the place, but all alternatives (some of which are discussed in STCInVOTableArchive, search for xtype) seemed worse to me. | |||||||
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< < | Impact on Functionality: | |||||||
> > | Change: DataModel.URI | |||||||
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< < | As far as I can see, None. You need one AstroCoords group per what
set of coordinates either way. -- MD
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> > | MD: Many people want some versioning of the data models, preferably connecting data models to URLs. After some consultation with Mireille, I propose to transmit this URL in DataModel.URI. It doesn't help to make instance documents much prettier but is IMHO a rather non-invasive method of making explicit what data model was used. | |||||||
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< < | I agree with this proposed change. As a matter of fact, it is the way STC was intended to function in VOTable (albeit as an imported schema, not through utypes). See the examples I post at the bottom of the page.
-- ArnoldRots - 01 Dec 2009
I agree and think that FIELDref-s SHOULD be used. Is most logical way to add extra information about a field. The utype on the field is then freed up for pointing into other possibly more samntically meaningful models, such as "is a position of a galaxy".
-- GerardLemson - 11 Mar 2010
Change 2: Flat systemsJust have all utype/value (belonging to one coordinate system definition) params as direct children of the AstroCoordSystem group.<GROUP ID="lltoush" utype="stc:AstroCoordSystem"> <PARAM arraysize="*" datatype="char" value="VELOCITY" utype="stc:AstroCoordSystem.RedshiftFrame.value_type" /> <PARAM arraysize="*" datatype="char" value="ICRS" utype="stc:AstroCoordSystem.SpaceFrame.CoordRefFrame" /> </GROUP> Rationale
Impact on FunctionalityNone that I can see. Were these groups meant a service for humans? -- MDCommentI don't think this will work, except for the simplest tables.It does not allow for multiple coordinate systems, reusing coordinate systems, or using elements that contain AstroCoordSystem elements. See the CSC example that I will be posting at the bottom of the page. -- ArnoldRots - 01 Dec 2009 Uh -- I notice I was not particularly clear. There is one group each for every AstroCoordSystem, of course. I'm just suggesting to drop the subgroups within (XFrame). For the CSC example, I can't see where that would fail, and actually, it should not in any setting, by virtue of the data model requriring zero or one of each XFrame and the frame name being a part of the utype already. -- MD 2009-12-02Change 3a: Do not abuse xml namespace declarationsDon't pretend the stc: in the utype has anything to do with an XML namespace. So, strike the xmlns:stc declaration on VOTABLE:<VOTABLE version="1.2" xmlns:xsi="http://www.w3.org/2 xmlns="http://www.ivoa.net/xml/VOTable/v1.2"> Rationale
Impact on FunctionalityThe VOTable has no way to define which version of the STC data model the utypes refer to. I would say this is desirable since versioned meanings will lead to hell either way, but see Change 3b for a fix. -- MDCommentI have no(t yet an) opinion on this. It does sound reasonable. -- ArnoldRots - 01 Dec 2009Change 3b: Define DM Version using UCDsIn every AstroCoordSystem group, declare what version of the DM you are using. We may make that optional or a strong recommendation or something like that. The version of the AstroCoords group would be implied via its ref.<GROUP ID="lltoush" utype="stc:AstroCoordSystem"> <PARAM utype="stc:" value="http://.../stc-v1.30#"/> <PARAM arraysize="*" datatype="char" value="VELOCITY" utype="stc:AstroCoordSystem.RedshiftFrame.value_type" /> Rationale
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CommentSounds reasonable; but you need a name and a datatype as well. -- ArnoldRots - 01 Dec 2009Change 4: Only allow string valuesDefine that all STC PARAMs aredatatype="char" arraysize="*" .
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CommentI haven't thought about the repercussions of this, yet. On the face of it, it sounds not unreasonable, but on the other hand, since the data type has to be given as a parameter, I don't see allowing more data types as much of a complication. I only wish that PARAM were more reasonable in the types it allows - particularly 'string' would be useful - and, of course, ISO-8601. -- ArnoldRots - 01 Dec 2009CSC Cone Serch ExamplesAs it so happened, I had recently prepared the STC-specific stuff for a VOTable 1.1/1.2 that presents data returned by a simple cone search query to the Chandra Source Catalog. Then I modified that one to comply with Changes 1 and 3 above. There is nothing like a real life example to bring out the problems![]() Revision, Draft 1I have prepared a revision of the note, and while doing so I realized that things become quite a bit simpler if the utype-value pairs are serialized into INFOs rather than PARAMs. Otherwise, it more or less reflects the changes proposed here. You check out the document from svn at http://svn.ari.uni-heidelberg.de/svn/gavo/stcvotable/trunk/ (read-only). For commit privileges, contact me. -- MarkusDemleitner - 21 Jan 2010Revision, Draft 2After some feedback from Arnold, I've prepared a second draft. Some contentious points remaining below. The current draft is at http://vo.ari.uni-heidelberg.de/docs/note_stc.pdf (and the (ugly) sources are still in the svn mentioned above). -- MarkusDemleitner - 17 Feb 2010Issues left in Draft 2STC containerArnold's suggestion is to give the AstroCoordSystem and the AstroCoords groups have a common parent. So, instead of having<group utype="stc:AstroCoordSystem" id="sys1"/> [other stuff] <group utype="stc:AstroCoords" ref="sys1"/>you would have <group utype="???"> <group utype="stc:AstroCoordSystem" id="sys1"/> <group utype="stc:AstroCoords" ref="sys1"/> </group>Still, constructs like <group utype="???"> <group utype="stc:AstroCoordSystem" id="sys1"/> </group> [other stuff] <group utype="???"> <group utype="stc:AstroCoords" ref="sys1"/> </group>would be allowed. Comment Markus: I don't really like this -- It creates an additional element for no apparent benefit since you still need to resolve the references. If, on the other hand, we'd abandon referencing completely, it would definitely be worth it, but people may resent the idea of not being able to reuse coordinate system definitions within a VOTable (though probably only a small fraction of the existing VOTables would actually suffer from not being able to do so). So, from me: Either referencing or top-level STC container. Comment Arnold: It keeps the STC stuff neatly together and if there ever is a need to add the observer's location, it can be done. It does make it easier to interpret the information in terms of an STC metadata object, which will come in handy when we finally have an STC library. And I don't particularly care for scattered metadata. CommentInventing an additional parent element doesn't look necessary to me - especially since you are still allowed to scatter the information and put the AstroCoords and AstroCoordSystem information separately. I vote for the version without a parent. -- KristinRiebe - 10 Mar 2010 Comment Arnold: I wasn't suggesting that scattering outside the containers be allowed, so Markus's second point does not hold water. -- ArnoldRots - 16 Mar 2010 Comment Markus: So, Arnold: You are fine with dropping the id/ref mechanism and forbid referencing of AstroCoordSystem groups across your container groups? I'd like that a lot, and in that case I'd propose to just flatly dump all utypes belonging to that coordinate system into one group. That would really make implementations a lot easier. What do the others think? -- 2010-03-31EpochShould an epoch like B1950.0 be encoded as<info utype="stc:AstroCoords.Position.Epoch" value="B1950.0"/>or as <info utype="stc:AstroCoords.Position.Epoch" value="1950.0"/> <info utype="stc:AstroCoords.Position.Epoch.whatever" value="B"/>Comment Arnold: Epoch is a number, not a string. If there were a limited number of values, one might consider to represent them with an enumerated list of strings, but that is not the case. It is a foolish hack to represent a numeric value with a string parameter; this a properly a numeric quantity with an attribute that says whether it is Julian or Besselian. Comment Markus: Splitting that perfectly understandable literal has negligible benefits at considerable cost. Plus, the votable schema already contains an appropriate type (astroYear). So, I can see no reason to double the amount of serialization and handling effort. CommentI agree with Markus - aren't Astronomical epochs practically always written with a leading character? So they should be defined as type astroYear and thus no confusion with numbers/strings can occur. Besides, it looks more concise and simpler to understand. -- KristinRiebe - 10 Mar 2010 Comment Arnold: This is a weak argument - why don't we consider everything a string and do away with all other datatypes? The fact that VOTable has a, in my opinion, poor representation for epochs does not mean that all other standards need to use the same. -- ArnoldRots - 16 Mar 2010 Comment Markus: We have types to denote the domains of variables and define the operators available for them. The domain of astronomical epochs represented by literals matching [JB][0-9]+(.[0-9]+) for quite some time now. By the way, even in the operators you see that astronomical epochs just aren't floats. If you say ~AstroCoords.Position.Epoch is a float, shouldn't I be perfectly entitled to just add them? And: What is the concrete utility of splitting the value? Does it reduce implementation effort? Will it help code correctness? --- 2010-03-31ReferencingShould the referencing between AstroCoords groups and AstroCoordSystem groups be done using VOTable referencing, viz.,<group utype="stc:AstroCoordSystem" id="sys1"/> [other stuff] <group utype="stc:AstroCoords" ref="sys1"/>or rather using utypes, viz., <group utype="stc:AstroCoordSystem"> <info utype="stc:AstroCoordSystem.id" value="sys1"/> </group> [other stuff] <group utype="stc:AstroCoords"> <info utype="stc:AstroCoords.coord_system_id" value="sys1"/> </group>Comment Arnold: If STC provides a referencing mechanism to tie its components together, that should be used to do so, not a VOTable mechanism. And if you encapsulate the whole thing in an STC container (see above), it is the natural, neatly self-contained way to do it. Comment Markus: STC doesn't really provide a referencing mechanism. There's some mechanism in STC-X but, e.g., none in STC-S. We really, really should use native referencing. Referencing is messy to get right without additional complications of having two different identifier systems (e.g., you need to get referential integrity and uniqueness right, and you need to catch cases when they are violated, and you need to tell the user that something went wrong, etc). Self-containedness is nice, but not at the cost of doubling the implementation effort in a tricky spot. So: Since we're writing VOTables, we should be using VOTable's referencing. CommentI vote for the first version - since VOTable's referencing system is doing a good job here, more complications (even if they could achieve self-containedness) are not necessary. -- KristinRiebe - 10 Mar 2010 Comment Arnold: With all dues respect, Markus's argument is incorrect. In STC-S the referencing is defined (implied) in the syntax and the referencing is part of the standard. Since we are connecting STC elements together, we should be using STC's referencing mechanism. -- ArnoldRots - 16 Mar 2010 Comment Markus: The argument about a significantly increased implementation effort is hardly incorrect -- you'll have to write all the code to handle the parallel referencing, don't you? But either way: Arnold, you still have not pointed out what the use of a second, parallel referencing is. Does it prevent errors? Will it help readers or writers? Does it decrease the file size? -- 2010-03-31Time utypesYet another thing I'd like to change is the representation of the encoding in the time utypes. As the standards is written now, there are utypes ...TimeInstant.ISOTime, ...TimeInstant.JDTime, and ...TimeInstant.MJDTime and analogously for some other such utypes. That is bad for at least three reasons:
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