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Space-Time Coordinate Metadata

Arnold RotsHarvard-Smithsonian CfA / CXC

THE US NATIONAL VIRTUAL OBSERVATORY

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Overview

• Justification: what is driving this?

• The simple requirements

• Design: how do we solve this?

• Implementation

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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

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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

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Requirement

• The requirement for Space-Time Coordinate metadata is that they:– provide sufficient and necessary information– are self-consistent

• For an observation two sets of metadata are required:– Location of the observatory– Location that the observation pertains to

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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

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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

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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, …

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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)

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Coordinate Area

• Defines the volume in coordinate space that is occupied by the Observation– Not necessarily relevant for VOEvent

• 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

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Implementations

• STC-X: XML schemata, current version:

1.20• STC-S:

String version• ObsDataLocation

ObservatoryLocationPosition GEO_D TOPOCENTER SPHER3 248.4056 31.9586 2158ObservationLocationTime UTC TOPOCENTER 2005-04-15T23:58:55 Error 5Position ICRS TOPOCENTER 148.88821 69.06529 Error 1.0 1.0Spectral TOPOCENTER 460 unit nm Resolution 40

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STC-X XML Example

• 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

• The event described was observed at KPNO in blue light, just before midnight tomorrow, and is known to be located within 1 degree of the nucleus of M81

• file:///EventKPNOinc.xml