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Space-Time Coordinate Metadata for VOTable

T HE US N ATIONAL V IRTUAL O BSERVATORY. Space-Time Coordinate Metadata for VOTable. Arnold Rots Harvard-Smithsonian CfA / CXC. Overview. Justification: what is driving this? The simple requirements Design: how do we solve this? Implementation Example. Scope.

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Space-Time Coordinate Metadata for VOTable

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  1. THE US NATIONAL VIRTUAL OBSERVATORY Space-Time Coordinate Metadata for VOTable Arnold Rots Harvard-Smithsonian CfA / CXC IVOA Interop Kyoto - VOTable

  2. Overview • Justification: what is driving this? • The simple requirements • Design: how do we solve this? • Implementation • Example IVOA Interop Kyoto - VOTable

  3. Scope • The following coordinate axes are closely intertwined: • Space • Time • Spectral (frequency, wavelength, energy) • Redshift (Doppler velocity) • Time is bound to a position and positions are time-variable • Spectral and redshift data are tied to reference frames that may or may not be time-variable IVOA Interop Kyoto - VOTable

  4. Scope (2) • Context-dependent defaults are fine • Issues are well-defined and clear for single-observatory observations – even when not all is explicitly specified • But there are no global defaults • In the VO all implicit assumptions need to be made explicit since they will not be “obvious” anymore • One must be able to transform the coordinates of two observations to a common coordinate system • Including far-field/near-field transformations IVOA Interop Kyoto - VOTable

  5. Requirement • The requirement for Space-Time Coordinate metadata is that they: • provide sufficient and necessary information • are self-consistent • We need to assure that coordinate transformations can be performed IVOA Interop Kyoto - VOTable

  6. The Metadata Components • Coordinate system • Consists of one or more frames • Frames typically consist of a reference position (origin) and a reference frame • Coordinate values • Refers to a coordinate system • Coordinate areas or ranges • To define a volume in coordinate space • Special case: Regions • Specifically for spatial coordinates IVOA Interop Kyoto - VOTable

  7. Coordinate System • Time frame • Reference position & time scale • Spatial frame • Reference position and coordinate frame • Spectral frame • Reference position in phase space • Redshift frame • Definition and reference position IVOA Interop Kyoto - VOTable

  8. Reference Frames and Positions • Examples of spatial Reference Frames: • FK4, FK5, ICRS, Ecliptic, Galactic, Geocentric, Geodetic, various solar and planetary frames, unknown, custom, … • Time scales: • TT, TAI, UTC, TDB, TEB, TCG, TCB, … • Reference Positions: • Topocenter, Geocenter, Heliocenter, Barycenter, Galactic center, LSR-K, LSR-D, planetary centers, unknown, custom, … IVOA Interop Kyoto - VOTable

  9. Coordinates • A coordinate object contains a reference to a coordinate system • It is a composite object that may hold • Name - Resolution • Value - Size • Error - Pixel size • All quantities are scalar except for spatial (>1D) • In that case errors and resolutions get more complicated • All coordinates include their units • Spatial may include position and velocity (PM) • Time has options (absolute or elapsed) IVOA Interop Kyoto - VOTable

  10. Coordinate Area • Defines the volume in coordinate space that is represented • Consists of one or more ranges in individual coordinates • Spatial position has more options • Sphere • 2-D Regions • Shapes: polygon, sector, ellipse, convex, convex hull • Operations: intersection, union, negation IVOA Interop Kyoto - VOTable

  11. Implementations • STC-X: XML schemata current version: 1.20 • STC-S: String version used in Resource Metadata IVOA Interop Kyoto - VOTable

  12. CatalogEntry A simple catalog of positions: <CatalogEntryLocation> <AstroCoordSystem> <TimeFrame/> <SpaceFrame/> </AstroCoordSystem> <AstroCoords> <Position2D/> </AstroCoords> <AstroCoordArea> <TimeInterval/> <PositionInterval/> </AstroCoordArea> <CatalogEntryLocation> IVOA Interop Kyoto - VOTable

  13. AstroCoordSystem • One could optionally add a SpectralFrame <AstroCoordSystem ID=“ICRS-TT-TOPO”> <TimeFrame> <TimeScale>TT</TimeScale> <TOPOCENTER> </TimeFrame> <SpaceFrame> <ICRS/> <TOPOCENTER/> <SPHERICAL coord_naxes=“2”/> </SpaceFrame> </AstroCoordSystem> IVOA Interop Kyoto - VOTable

  14. AstroCoordSystem (2) • The coordinate system may also be specified through an XInclude: <xi:include href=http://www.ivoa.net/xml/STC/ICRS-TT-TOPO.xml/> IVOA Interop Kyoto - VOTable

  15. AstroCoords • The position vector points to an element with ID=“Column3” – presumably a Field in the document that describes column 3 of the table; in addition, an error is given <AstroCoords coord_system_id=“ICRS-TT-TOPO”> <Position2D unit=“deg”> <Value2Ref>Column3</Value2Ref> <Error2Radius>0.0002</Error2Radius> </Position2D> </AstroCoords> IVOA Interop Kyoto - VOTable

  16. AstroCoordArea • This element is not required, but it is a mechanism to provide information on the coverage of the catalog <AstroCoordArea> <TimeInterval> <StartTime> <ISOTime>2000-01-01T00:00:00</ISOTime> </StartTime> <StopTime> <ISOTime>2005-01-01T00:00:00</ISOTime> </StopTime> </TimeInterval> IVOA Interop Kyoto - VOTable

  17. AstroCoordsArea (2) • Time interval is only for illustrative purposes; only position may be useful; one could optionally add a spectral interval <PositionInterval unit=“deg”> <Coord2VecInterval> <LoLimit2Vec>0 0</LoLimit2Vec> <HiLimit2Vec>90 30</HiLimit2Vec> </Coord2VecInterval> </PositionInterval> </AstroCoordArea> IVOA Interop Kyoto - VOTable

  18. XInclude • The physical XML documents can be simplified through use of XInclude files that allow inclusion of frequently used elements, referenced through standardized IDREFs • Constructors can just insert the single lines • Parsers that do not care about coordinate systems or observatory positions can just ignore them and optionally rely on the IDREFs, while the document remains rigorously correct IVOA Interop Kyoto - VOTable

  19. Extensibility • This was a simple example; the system can be extended to handle more sophisticated catalogs: • Multiple coordinate systems (e.g., l,b) • More coordinate axes: • Proper motions • Redshifts, radial velocities • Spectral information • Temporal information • Additional information: errors, sizes, etc. • Merged catalogs IVOA Interop Kyoto - VOTable

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