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2005-05-20IVOA Interop Kyoto - DM 1
Space-Time Coordinate Metadata: Status
Arnold RotsHarvard-Smithsonian CfA / CXC
THE US NATIONAL VIRTUAL OBSERVATORY
2005-05-20IVOA Interop Kyoto - DM 2
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
2005-05-20IVOA Interop Kyoto - DM 3
Implementations
• STC-X: XML schematacurrent version: 1.20
• STC-S: String versionused in Resource Metadata
2005-05-20IVOA Interop Kyoto - DM 4
Status
• STC is in the Proposed Recommendation stage• A number of (mostly minor) points came up at
the VOEvent meeting, mainly affecting STC-X implementation
• More comments came from VOTable• Currently being incorporated in
Version 1.22• Coordination with Characterization• Coordination with Quantity
2005-05-20IVOA Interop Kyoto - DM 5
Enhancements
• Name in coordinates optional• TimeScale optional in AstroTime• XInclude examples• AstroCoordArea optional for ObsLocation• Error circles• Epoch• IDs
• STC-S has been incorporated in Resource Metadata and SkyNode
• David Berry is hard at work on coordinate transformations using AST
2005-05-20IVOA Interop Kyoto - DM 6
Musings
• It would really be good to have an accepted set of XML Schema best practices for IVOA
• Establishment of a review board for XML schemata?
2005-05-20IVOA Interop Kyoto - DM 7
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
2005-05-20IVOA Interop Kyoto - DM 8
XInclude Issues
• It’s a beautiful idea to pre-define coordinate systems, but is it worth it?– SpaceFrame: ICRS, FK4, FK4, GAL, SGAL, ECL, GEOD,
GEOC– SpaceReference: TOPO, BARY, GEO– TimeFrame: TT, UTC– TimeReference: TOPO, BARY, GEO– SpectralReference: TOPO, GEO, BARY– Redshift: OPT, RAD; Z, DOPVEL– RedshiftReference: TOPO, GEO, BARY, LSR
• Adds up to > 800
2005-05-20IVOA Interop Kyoto - DM 9
Issues to be Resolved
• What Frames are required (Time & Space?)• Handling of namespaces• Should Observatory and Observation Location
be uncoupled?• Should multiple Observations and/or multiple
coordinates be allowed?• IDs for all leaves, allowing linking in either
direction• Unions versus Substitution Groups versus
xsi:type – general XML schema best practices questions
• Grey area between error box and Region
2005-05-20IVOA Interop Kyoto - DM 10
Overview
• Justification: what is driving this?
• The simple requirements
• Design: how do we solve this?
• Implementation
• Next - issues
2005-05-20IVOA Interop Kyoto - DM 11
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
2005-05-20IVOA Interop Kyoto - DM 12
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
2005-05-20IVOA Interop Kyoto - DM 13
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
2005-05-20IVOA Interop Kyoto - DM 14
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
2005-05-20IVOA Interop Kyoto - DM 15
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, …
2005-05-20IVOA Interop Kyoto - DM 16
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)
2005-05-20IVOA Interop Kyoto - DM 17
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
2005-05-20IVOA Interop Kyoto - DM 18
CatalogEntry
A simple catalog of positions:<CatalogEntryLocation>
<AstroCoordSystem><TimeFrame/><SpaceFrame/>
</AstroCoordSystem><AstroCoords>
<Position2D/></AstroCoords><AstroCoordArea>
<TimeInterval/><PositionInterval/>
</AstroCoordArea><CatalogEntryLocation>
2005-05-20IVOA Interop Kyoto - DM 19
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>
2005-05-20IVOA Interop Kyoto - DM 20
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/>
2005-05-20IVOA Interop Kyoto - DM 21
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><Position2D unit=“deg”>
<Value2Ref>Column3</Value2Ref><Error2Radius>0.0002</Error2Radius>
</Position2D></AstroCoords>
2005-05-20IVOA Interop Kyoto - DM 22
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>
2005-05-20IVOA Interop Kyoto - DM 23
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>
2005-05-20IVOA Interop Kyoto - DM 24
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
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