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Space-Time Coordinate Metadata RFC

This document will act as RFC centre for the Space-Time Metadata V1.21 Proposed Recommendation.

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 WikiName so 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.

Discussion about any of the comments or responses should be conducted on the data model mailing list, dm@ivoa.net.

Comments

  • (1) The following comments affecting the document came from the VOEvent workshop:
    • Can Name in Coordinate be made optional?
    • What Frames should really be required in AstroCoordSystem?
    • Can the duplication of TimeScale in AstroCoordSystem and AstronTime be avoided?
    • XInclude examples in Appendix C?
-- ArnoldRots - 16 May 2005

  • (2) The following comments affecting the STC-X (schema) implementation came from the VOEvent workshop:
    • Can Observatory and Observation Locations be uncoupled?
    • Can multiple Observations and multiple coordinates be accommodated in ObsDataLocation?
    • Can AstroCoordArea be made optional in ObservationLocation?
    • Can there be an simpler description for error circles in the case of 2-D spatial coordinate systems?
-- ArnoldRots - 16 May 2005

  • (3) I'm concerned about the amount of "implementation" that has been done to date. Could we have a definitive statement of what is serving as our 2 implementations?
    I don't think having some example XML instance documents is sufficient for serving as an "implementation". We need actual software that actually does something with it. Use of STC in SkyNode does qualify; however, I think it is fair to say that this is only partial use. Something like the footprint service would perhaps provide more coverage of the schema. A stylesheet that converts STC into an astronomer-friendly description of a coordinate system, region, etc. would also be useful. It's not until a developer (because users are going to look at this stuff) actually sits down with STC and tries to use it will we understand if the schema does the job.
-- RayPlante - 16 May 2005

  • (4) From the VOTable discussion came the following comments:
    • Epoch is needed, especially for positions corrected for proper motion to a date other than the observing date.
    • Would it be possible to link from the table to the STC element(s), rather than the other way around?
    • Can we leave metadata out if they're not relevant or unknown?
-- ArnoldRots - 17 May 2005


Responses

  • (1) The four requests from VOEvent affecting the document are reasonable and I will amend the document accordingly. The corresponding modifications to the schema implementation are minor and do not affect the validity of existing documents.
Specifically:
    • CoordName in Section 4.3.1 will be made optional.
    • SpectralFrame will be dropped from the list of required frames for AstroCoordSystem in Section 4.5.
    • TimeScale will be made optional in AstronTime in Section 4.6.1.1, though it will be stressed that it should only be omitted if that element is associated with and AstroCoordSystem.
    • I will suggest a standard naming of XInclude files and the IDs they contain, to reflect their contents and provide examples for ICRS-TT-TOPO and FK5-UTC-TOPO as representing commonly used cases.
-- ArnoldRots - 16 May 2005

  • (2) As to the four comments from VOEvent on the schema implementation: the last two are minor and will be implemented, while the first two need additional study; none of them affect the PR or existing documents based on the schemata.
-- ArnoldRots - 16 May 2005

  • A few typos were pointed out and a suggestion for improvement (use of different units) was made for the example in Appendix B.4: The coord_system_id should be "ICRS-TT-TOPO", the wavelength units might be changed to "nm", the Start and Stop Times need to be corrected, and the pixel number upper limits need to be corrected. These changes will be made.
-- ArnoldRots - 16 May 2005

  • A query from the Catalog group made me realize that 3-D spherical coordinates need to be extended to allow "deg deg Mpc" as units in Section 4.5.2.2; this will be accommodated.
-- ArnoldRots - 16 May 2005

  • (3) There are two applications that I am currently aware of: David Berry has built STC support into AST and there is a partial implementation in SkyNode, as RayPlante pointed out.
-- ArnoldRots - 17 May 2005

  • (4) The need for an epoch is acknowledged and will be added in Section 4.5.2.3.
    The direction of the linking is only an issue in the schema, not the document. The reverse linking will be accommodated by providing the "leaves" of the coordinate element with ID attributes and making them nillable. My main concern is whether a document with reverse linking is robust in terms of ensuring sufficient (and consistent) information to allow execution of coordinate transformations.
    It is dangerous for a service provider to claim being able to fathom all the needs of all potential users, deciding what metadata they do need and what they don't. It may be correct in many cases, but there are also many in the grey area where the research interests of data providers may color their perception of their users' needs. It is far better to provide a complete set of metadata in all situations. And STC provides for UNKNOWN as a legitimate value in those cases where it is appropriate.
-- ArnoldRots - 17 May 2005



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Topic revision: r6 - 2005-05-17 - ArnoldRots
 
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