International

Virtual

Observatory

Alliance

Space-Time Coordinate (STC) Metadata Model

Version 1.31

IVOA Note 5 December 2007

This version:

http://www.ivoa.net/Documents/Notes/STC/STC-Model-20071205.html

Latest version:

http://www.ivoa.net/Documents/latest/STC-Model.html

Previous version(s):

http://www.ivoa.net/Documents/Notes/STC/STC-Model-20071030.html

Author(s):

A. H. Rots

Abstract

This document provides a summary of the model for the description of the VO Space-Time Coordinate metadata. It shows the hierarchical relation between the various components with brief descriptions where needed. The Tables that are referred to may be found in the STC document. XML element names are shown in bold Courier font.

The objective is to provide a framework that allows a complete and internally consistent specification of coordinate metadata – primarily the intertwined temporal, spatial, spectral, and redshift coordinates – that is extensible to accommodate all future applications.

Status of This Document

This is a Note. The first release of this document was 2007-12-05.

This is an IVOA Note expressing suggestions from and opinions of the authors. It is intended to share best practices, possible approaches, or other perspectives on interoperability with the Virtual Observatory. It should not be referenced or otherwise interpreted as a standard specification.

A list of current IVOA Recommendations and other technical documents can be found at http://www.ivoa.net/Documents/.

Table of Contents

1 STC Metadata Description 6

1.1 Coordinate System (CoordSys, AstroCoordSystem, PixelCoordSystem) 6

1.1.1 Generic Coordinate Frame (CoordFrame) 7

1.1.1.1 Reference Position (CoordRefPos) 7

1.1.1.2 Generic Coordinate Reference Frame (CoordRefFrame) 7

1.1.1.2.1 ScalarRefFrame 7

1.1.1.2.2 Cart2DRefFrame 7

1.1.1.2.3 Cart3DRefFrame 7

1.1.1.2.4 SphericalRefFrame 7

1.1.1.3 Coordinate Flavor (CoordFlavor) 7

1.1.2 Time Frame (TimeFrame) 8

1.1.2.1 Reference Position (ReferencePosition) 8

1.1.2.1.1 Standard Reference Position 8

1.1.2.1.1.1 Planetary Ephemeris (PlanetaryEphem; conditional) 8

1.1.2.1.2 Custom Reference Position (CoordRefPos) 8

1.1.2.2 Time Scale 8

1.1.2.3 Time Reference Direction (TimeRefDirection; conditional) 8

1.1.3 Space Frame (SpaceFrame) 9

1.1.3.1 Reference Position (ReferencePosition) 9

1.1.3.1.1 Standard Reference Position 9

1.1.3.1.1.1 Planetary Ephemeris (PlanetaryEphem; conditional) 9

1.1.3.1.2 Custom Reference Position (CoordRefPos) 9

1.1.3.2 Space Reference Frame 9

1.1.3.2.1 Standard Reference Frame 9

1.1.3.2.1.1 Equinox (conditional) 9

1.1.3.2.2 (Custom) Coordinate Reference Frame (CoordRefFrame) 9

1.1.3.2.2.1 ScalarRefFrame 9

1.1.3.2.2.2 Cart2DRefFrame 10

1.1.3.2.2.3 Cart3DRefFrame 10

1.1.3.2.2.4 SphericalRefFrame 10

1.1.3.3 Coordinate Flavor (CoordFlavor) 10

1.1.3.4 Offset Center (OffsetCenter; optional) 10

1.1.4 Spectral Frame (SpectralFrame) 11

1.1.4.1 Reference Position 11

1.1.4.1.1 Standard Reference Position 11

1.1.4.1.1.1 Planetary Ephemeris (PlanetaryEphem; conditional) 11

1.1.4.1.2 Custom Reference Position (CoordRefPos) 11

1.1.5 Redshift Frame (RedshiftFrame) 12

1.1.5.1 Reference Position 12

1.1.5.1.1 Standard Reference Position 12

1.1.5.1.1.1 Planetary Ephemeris (PlanetaryEphem; conditional) 12

1.1.5.1.2 Custom Reference Position (CoordRefPos) 12

1.1.5.2 Doppler Definition (DopplerDefinition) 12

1.1.6 Pixel Frame (PixelFrame) 13

1.1.6.1 Reference Position (CoordRefPos) 13

1.1.6.2 Coordinate Reference Frame (CoordRefFrame) 13

1.1.6.2.1 ScalarRefFrame 13

1.1.6.2.2 Cart2DRefFrame 13

1.1.6.2.3 Cart3DRefFrame 13

1.1.6.2.4 SphericalRefFrame 13

1.1.6.3 Coordinate Flavor (CoordFlavor) 13

1.1.6.4 Reference Pixel (ReferencePixel) 13

1.2 Coordinates (Coords, AstroCoords, PixelCoords) 14

1.2.1 Generic Coordinate (GenCoordinate) 15

1.2.1.1 ScalarCoordinate 15

1.2.1.2 Vector2DCoordinate 15

1.2.1.3 Vector3DCoordinate 15

1.2.1.4 StringCoordinate 15

1.2.2 Time (Time) 16

1.2.2.1 Absolute Time (AbsoluteTime) 16

1.2.2.2 Time Offset (TimeOffset; optional) 16

1.2.3 Spatial Position (Position) 17

1.2.3.1 Position1D 17

1.2.3.2 Position2D 17

1.2.3.3 Position3D 17

1.2.4 Spatial Velocity (Velocity) 17

1.2.4.1 Velocity1D 17

1.2.4.2 Velocity2D; 17

1.2.4.3 Velocity3D; 17

1.2.5 Orbital Elements (Orbit; alternative) 18

1.2.5.1 Semi-major axis (a; conditional) 18

1.2.5.2 Periapsis Distance (q; conditional) 18

1.2.5.3 Eccentricity (e) 18

1.2.5.4 Inclination (i) 18

1.2.5.5 Longitude of Ascending Node (Node) 18

1.2.5.6 Argument of Periapsis (Aop) 18

1.2.5.7 Mean Anomaly (M; optional) 18

1.2.5.8 Orbital Period (P; optional) 18

1.2.5.9 Epoch (T) 18

1.2.6 Spectral Coordinate (Spectral) 19

1.2.7 Redshift Coordinate (Redshift) 19

1.2.8 Coordinate File (CoordFile) 20

1.2.8.1 FITSFile 20

1.2.8.2 FITSTime (optional) 20

1.2.8.3 FITSPosition (optional) 20

1.2.8.4 FITSVelocity (optional) 20

1.2.8.5 FITSSpectral (optional) 20

1.2.8.6 FITSRedshift (optional) 20

1.2.9 Pixel Coordinate (Pixel) 21

1.2.9.1 Pixel1D 21

1.2.9.2 Pixel2D 21

1.2.9.3 Pixel3D 21

1.3 Coordinate Area (CoordArea, AstroCoordArea, PixelCoordArea) 22

1.3.1 Generic Scalar Range (CoordInterval) 23

1.3.1.1 CoordScalarInterval 23

1.3.1.2 Coord2VecInterval 23

1.3.1.3 Coord3VecInterval 23

1.3.2 Time Interval (TimeInterval) 23

1.3.3 Spatial Area (PositionInterval) 24

1.3.3.1 1-Dimensional Intervals (PositionScalarInterval) 24

1.3.3.2 2-Dimensional Rectangles (Coord2VecInterval) 24

1.3.3.3 2-Dimensional Region 24

1.3.3.3.8 ConvexHull 25

1.3.3.3.9 SkyIndex 25

1.3.3.3.10 Union 25

1.3.3.3.11 Intersection 25

1.3.3.3.12 Negation 25

1.3.3.3.13 Difference 25

1.3.3.4 3-Dimensional Intervals (Coord3VecInterval) 25

1.3.3.5 3-Dimensional Sphere (Sphere) 25

1.3.4 Spatial Velocity 26

1.3.4.1 1-Dimensional Intervals (VelicityScalarInterval) 26

1.3.4.2 2-Dimensional Rectangles (Velocity2VecInterval) 26

1.3.4.3 3-Dimensional Intervals (Velocity3VecInterval) 26

1.3.4.4 3-Dimensional Sphere (VelocitySphere) 26

1.3.5 Spectral Interval (SpectralInterval) 27

1.3.6 Redshift Interval (RedshiftInterval) 27

1.3.7 Pixel Interval (PixelCoordInterval) 27

1.3.7.1 PixelCoordScalarInterval 27

1.3.7.2 PixelCoord2VecInterval 27

1.3.7.3 PixelCoord3VecInterval 27

2 Units 28

2.1 Time Units 28

2.2 Spectral Coordinate Units 28

2.2.1 Frequency 28

2.2.2 Wavelength 28

2.2.3 Energy 28

2.3 Spatial Coordinate Units 28

2.3.1 Angular Units 28

2.3.2 Linear Units 29

2.3.3 Three-units Strings 29

2.3.4 Position Angles 29

3 Usage 30

3.1 Referencing 30

3.2 Use Context 30

3.2.1 Resource Profile (STCResourceProfile) 30

3.2.2 Search (SearchLocation) 30

3.2.3 Catalog Data (CatalogEntryLocation) 30

3.2.4 Observational Data (ObsDataLocation) 30

4 Global Elements 31

1 STC Metadata Description

An STC metadata description element consists of three components:

Coordinate System
Coordinates

3. Coordinate area

Missing components or subcomponents shall be considered UNKNOWN; this is to be interpreted as: it is up to the client to decide whether or not to accept the metadata, and to assign a sensible default value if the component is relevant.

1.1 Coordinate System (CoordSys, AstroCoordSystem, PixelCoordSystem)

A Coordinate System consists of one or more Coordinate Frames.

A Coordinate Frame typically consists of a Reference Frame (orientation) and a Reference Position (or Origin), but some Frames contain more, or less, information. There are six kinds of Frames.

The three kinds of Coordinate System are:

(Generic) CoordSys; contains:

One or more Generic Coordinate Frames

AstroCoordSystem; may contain:

One or more Generic Coordinate Frames
One Time Frame
One Space Frame
One Spectral Frame
One Redshift Frame

PixelCoordSystem; may contain:

One or more Generic Coordinate Frames
One or more Pixel Coordinate Frames

1.1.1 Generic Coordinate Frame (CoordFrame)

Allows specifying metadata for non-STC coordinates.

A CoordFrame contains a Coordinate Flavor and may contain a Coordinate Reference Frame and/or a Coordinate Reference Position; it requires a frame_id.

1.1.1.1 Reference Position (CoordRefPos)

A CoordRefPos consists of a Coordinate.

1.1.1.2 Generic Coordinate Reference Frame (CoordRefFrame)

There are four types of Coordinate Reference Frame:

(All Transformation elements contain a projection attribute, using values from Table 4)

1.1.1.2.1 ScalarRefFrame

Contains:

Scale

1.1.1.2.2 Cart2DRefFrame

Contains one of:

Transform2
Transform2Matrix

1.1.1.2.3 Cart3DRefFrame

Contains one of:

Transform3
Transform3Matrix

1.1.1.2.4 SphericalRefFrame

Contains:

Frame
Pole_Zaxis
Xaxis

1.1.1.3 Coordinate Flavor (CoordFlavor)

CARTESIAN, SPHERICAL, UNITSPHERE, POLAR, CYLINDRICAL, HEALPIX, or STRING; contains: dimensionality.

1.1.2 Time Frame (TimeFrame)

A TimeFrame contains a Reference Position and a Time Scale, and may contain a Time Reference Direction.

1.1.2.1 Reference Position (ReferencePosition)

A Reference Position may be a Standard or a Custom one:

1.1.2.1.1 Standard Reference Position

TOPOCENTER, GEOCENTER, BARYCENTER, etc., as in Table 1, noting the exceptions.

All Solar System positions, except GEOCENTER, should include a Planetary Ephemeris.

1.1.2.1.1.1 Planetary Ephemeris (PlanetaryEphem; conditional)

JPL-DE200 or JPL-DE405; needed when such am ephemeris is used to transform times.

1.1.2.1.2 Custom Reference Position (CoordRefPos)

A Coordinate Reference Position consists of a Coordinate.

1.1.2.2 Time Scale

TT (TDT), ET, TAI (IAT), UTC, GPS, TDB (TEB), TCG, TCB, LST, LOCAL (Table 2).

1.1.2.3 Time Reference Direction (TimeRefDirection; conditional)

If Time Reference Position is not TOPOCENTER, this direction (a Coordinate element) needs to be provided.

1.1.3 Space Frame (SpaceFrame)

A SpaceFrame includes a Reference Position, Spatial Reference Frame, and Coordinate Flavor, and may include an Offset Center.

1.1.3.1 Reference Position (ReferencePosition)

A Reference Position may be a Standard or a Custom one:

1.1.3.1.1 Standard Reference Position

TOPOCENTER, GEOCENTER, BARYCENTER, etc., as in Table 1, noting the exceptions (but including RELOCATABLE).

All Solar System positions, except GEOCENTER, should include a Planetary Ephemeris.

1.1.3.1.1.1 Planetary Ephemeris (PlanetaryEphem; conditional)

JPL-DE200 or JPL-DE405; needed when such an ephemeris is used to transform times.

1.1.3.1.2 Custom Reference Position (CoordRefPos)

A Coordinate Reference Position consists of a Coordinate.

1.1.3.2 Space Reference Frame

A Space Reference Frame may be a Standard or a Custom one:

1.1.3.2.1 Standard Reference Frame

FK4, FK5, ICRS, ECLIPTIC, GALACTIC, etc., as in Table 3, including UNKNOWNFrame.

1.1.3.2.1.1 Equinox (conditional)

Bnnnn or Jnnnn, for FKi only

1.1.3.2.2 (Custom) Coordinate Reference Frame (CoordRefFrame)

There are four types of Coordinate Reference Frame:

(All Transformation elements contain a projection attribute, using values from Table 4)

1.1.3.2.2.1 ScalarRefFrame

Contains:

Scale

1.1.3.2.2.2 Cart2DRefFrame

Contains one of:

Transform2
Transform2Matrix

1.1.3.2.2.3 Cart3DRefFrame

Contains one of:

Transform3
Transform3Matrix

1.1.3.2.2.4 SphericalRefFrame

Contains:

Frame
Pole_Zaxis
Xaxis

1.1.3.3 Coordinate Flavor (CoordFlavor)

CARTESIAN, SPHERICAL, UNITSPHERE, POLAR, CYLINDRICAL, or HEALPIX; contains: dimensionality.

1.1.3.4 Offset Center (OffsetCenter; optional)

For offset coordinates; consists of a Coordinate.

1.1.4 Spectral Frame (SpectralFrame)

A SpectralFrame only contains a Reference Position

1.1.4.1 Reference Position

Note that the Spectral Reference Position requires position as well as a velocity vector. A Reference Position may be a Standard or a Custom one:

1.1.4.1.1 Standard Reference Position

TOPOCENTER, GEOCENTER, BARYCENTER, etc., as in Table 1, noting the exceptions (but including LSRx).

All Solar System positions, except GEOCENTER, should include a Planetary Ephemeris.

1.1.4.1.1.1 Planetary Ephemeris (PlanetaryEphem; conditional)

JPL-DE200 or JPL-DE405; needed when such am ephemeris is used to transform times.

1.1.4.1.2 Custom Reference Position (CoordRefPos)

A Coordinate Reference Position consists of a Coordinate.

1.1.5 Redshift Frame (RedshiftFrame)

A RedshiftFrame contains a Reference Position and a Doppler Definition.

It also contains an attribute that indicates whether the coordinate values represent redshifts or Doppler velocities.

1.1.5.1 Reference Position

Note that the Redshift Reference Position also requires position as well as a velocity vector. A Reference Position may be a Standard or a Custom one:

1.1.5.1.1 Standard Reference Position

TOPOCENTER, GEOCENTER, BARYCENTER, etc., as in Table 1, noting the exceptions (but including LSRx).

All Solar System positions, except GEOCENTER, should include a Planetary Ephemeris.

1.1.5.1.1.1 Planetary Ephemeris (PlanetaryEphem; conditional)

JPL-DE200 or JPL-DE405; needed when such am ephemeris is used to transform times.

1.1.5.1.2 Custom Reference Position (CoordRefPos)

A Coordinate Reference Position consists of a Coordinate.

1.1.5.2 Doppler Definition (DopplerDefinition)

Allowed values: OPTICAL, RADIO, RELATIVISTIC.

1.1.6 Pixel Frame (PixelFrame)

A PixelFrame contains a Coordinate Flavor and may contain a Coordinate Reference Frame, a Coordinate Reference Position, and/or a Reference Pixel. It contains one or more axis_order attributes.

1.1.6.1 Reference Position (CoordRefPos)

A CoordRefPos consists of a Coordinate.

1.1.6.2 Coordinate Reference Frame (CoordRefFrame)

There are four types of Coordinate Reference Frame:

(All Transformation elements contain a projection attribute, using values from Table 4)

1.1.6.2.1 ScalarRefFrame

Contains:

Scale

1.1.6.2.2 Cart2DRefFrame

Contains one of:

Transform2
Transform2Matrix

1.1.6.2.3 Cart3DRefFrame

Contains one of:

Transform3
Transform3Matrix

1.1.6.2.4 SphericalRefFrame

Contains:

Frame
Pole_Zaxis
Xaxis

1.1.6.3 Coordinate Flavor (CoordFlavor)

CARTESIAN, SPHERICAL, UNITSPHERE, POLAR, CYLINDRICAL, HEALPIX, or STRING; contains: dimensionality.

1.1.6.4 Reference Pixel (ReferencePixel)

ReferencePixel contains a 1, 2, or 3-dimensional Pixel.

1.2 Coordinates (Coords, AstroCoords, PixelCoords)

A Coordinates element consists of up to six Coordinate elements and contains a reference to a Coordinate System. Each Coordinate has five components. Each component may occur once (a definite or typical value) or twice (indicating a range of values):

Value
Error
Resolution
Size
Pixel size

All components are optional; in addition, there may be a name. One or more Coordinate elements may be provided through a binary FITS table.

A variety of error types may be defined; statements below on the data types of errors are to be considered preliminary, contingent on additional definitions.

There are nine types of Coordinate elements, each of which is a composite of the above-mentioned components, except where noted.

Each Coordinates object needs to refer to a Coordinate System through a coord_system_id attribute.

There are three types of Coordinates elements:

(Generic) Coords; contains one or more Generic Coordinates and refers to a CoordSys; individual Coordinates need to refer to a specific Coordinate Frame through a frame_id
AstroCoords; refers to an AstroCoordSystem and may contain:

One or more Generic Coordinates; individual Coordinates need to refer to a specific Coordinate Frame through a frame_id
One Time Coordinate
One Spatial Position Coordinate
One Spatial Velocity Coordinate
One Orbit Coordinate
One Spectral Coordinate
One Redshift Coordinate
One Coordinate File

PixelCoords; refers to an PixelCoordSystem and may contain one or more:

Generic Coordinate
Pixel Coordinate

Individual Coordinates need to refer to a specific Coordinate Frame through a frame_id.

1.2.1 Generic Coordinate (GenCoordinate)

The Coordinate needs to refer to a specific Coordinate Frame through a frame_id. There are four types of GenCoordinate:

1.2.1.1 ScalarCoordinate

Optional units; all components are scalar doubles.

1.2.1.2 Vector2DCoordinate

Optional units; Error, Resolution, Size, PixSize may consist of:

Two doubles
2x2 matrix
Radius

1.2.1.3 Vector3DCoordinate

Optional units; Error, Resolution, Size, PixSize may consist of:

Three doubles
3x3 matrix
Radius

1.2.1.4 StringCoordinate

Only contains a (string) Value, no other components.

1.2.2 Time (Time)

The value (TimeInstant) is an instant of the AstronTime class; all other components (Error, Resolution, Size, PixSize) are doubles and need a unit (s, d, a or yr, cy). AstronTime contains an Absolute Time element and an optional relative time element. It also may have a TimeScale attribute.

AstronTime contains the following:

1.2.2.1 Absolute Time (AbsoluteTime)

Absolute Time may be of type ISOTime (ISO-8601 format), JDTime, or MJDTime. This should be a decimal for JD and MJD since a double is not guaranteed to have sufficient accuracy. In addition, for use with simulations, it may be of the type TimeOrigin which currently may only have the value RELOCATABLE.