Space-Time Coordinate Metadata for VOTable

2005-05-16IVOA Interop Kyoto - VOTable 1

Space-Time Coordinate Metadata for VOTable

Arnold RotsHarvard-Smithsonian CfA / CXC

THE US NATIONAL VIRTUAL OBSERVATORY


Overview

• Justification: what is driving this?

• The simple requirements

• Design: how do we solve this?

• Implementation

• Example


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


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


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


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


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


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, …


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)


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


Implementations

• STC-X: XML schematacurrent version: 1.20

• STC-S: String versionused in Resource Metadata


CatalogEntry

A simple catalog of positions:<CatalogEntryLocation>

<AstroCoordSystem><TimeFrame/><SpaceFrame/>

</AstroCoordSystem><AstroCoords>

<Position2D/></AstroCoords><AstroCoordArea>

<TimeInterval/><PositionInterval/>

</AstroCoordArea><CatalogEntryLocation>


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>


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/>

http://www.ivoa.net/xml/STC/ICRS-TT-TOPO.xml


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>


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>


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>


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


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

Space-Time Coordinate Metadata for VOTable

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