Space-Time Coordinate Metadata for the Virtual Observatory
Version 1.0
IVOA WG Internal Draft 2004-07-21
Working Group:
http://www.ivoa.net/twiki/bin/view/IVOA/IvoaDataModel
Authors:
Arnold Rots – Harvard-Smithsonian Center for Astrophysics
Abstract
This document provides a complete design description of the Space-Time Coordinate metadata for the Virtual Observatory. It explains the various components, highlights some implementation considerations, presents a complete set of UML diagrams, and discusses the relation between STC and certain other parts of the Data Model.
Status of this document
This is a Working Group Internal Draft
Acknowledgements
I gratefully acknowledge many helpful discussions with Wil O’Mullane, Alex Szalay, Jonathan McDowell, Ray Plante, Ed Shaya, Pat Dowler, and many others. I am indebted to SAO, ULP, CDS, and the NSF NVO grant for support.
Table of Contents
Space-Time Coordinate Metadata for the Virtual Observatory
IVOA WG Internal Draft 2004-07-21
Table of Contents1. Introduction
3. Requirements and Use-Context
4.2 Coordinate System (CoordSys)
4.2.1 Coordinate Frame (CoordFrame)
4.4 Coordinate Area (CoordArea)
Table 1. Standard Reference Positions
Table 3. Standard Reference Frames
Appendix A – UML Representation
Appendix C - XInclude Library of Coordinate Systems
Appendix D - Changes from Previous Versions
1. Introduction
This document attempts to explain and document the design and implementation of the Space-Time (and related coordinate axes) metadata for the Virtual Observatory. In order to make this specification optimally useful for the VO, we have generalized the Coordinate System concept and we have added a generic Coordinate Frame. Derived from these is the AstroCoordSys and the time, space, redshift, spectral frames that together make up the STC coordinate system. Coordinate and Coordinate Area also have generic as well as Astro versions. There are five companion files to this document:
- http://hea-www.harvard.edu/~arots/nvometa/STC_UML.pdf
presents the STC metadata design described in this document as UML diagrams; it is also reproduced in the appendix to this document. - http://www.ivoa.net/xml/STC/v1.0
http://hea-www.harvard.edu/~arots/nvometa/stc.xsd
is the top-level schema of the XML implementation, containing the coordinate system and coordinate area definitions. - http://www.ivoa.net/xml/STCcoords/v1.0
http://hea-www.harvard.edu/~arots/nvometa/coords.xsd
is the schema of the XML implementation with the coordinate value definitions. - http://www.ivoa.net/xml/STCregion/v1.0
http://hea-www.harvard.edu/~arots/nvometa/region.xsd
is the schema of the XML implementation for the region specification. - http://hea-www.harvard.edu/~arots/nvometa/STCPoster.pdf
is a poster showing a slightly outdated, but still useful, version of the entire STC schema.
The schemas are now fully up-to-date.
In the following sections we shall discuss the justification and scope for this metadata specification; the requirements and use context; a detailed design description; and some implementation considerations. Some concepts are only provided in their simplest form and we expect that more sophisticated classes will be derived (and will fit in seamlessly) as work progresses. The emphasis of this document is on the framework of the STC metadata.
Note:
In order to simplify the XML schemata, we decided as of version 0.5 to forego the capability of specifying Units for each component of the Coordinate element (see Sections 4.3.1, 4.4, and 4.6) and the option of specifying spatial position coordinates in sexagesimal format (see sections 4.6.2 and 4.6.2.3).
2. Justification and Scope
Before we plunge into the requirements and design of these metadata structures, it may be helpful to ask what is driving this exercise and to provide some context to the design. Our intent in this is to explain why the design needs to be fully general and extensible, leading to its alleged complexity.
The basic justification for considering the following coordinate axes in a single metadata specification is that they are closely intertwined:
- Space (including its time-derivatives, i.e., velocities)
- Time
- Spectral (frequency, wavelength, energy)
- Redshift (Doppler velocity)
It should be emphasized that the STC design distinguishes between the spectral coordinate and the redshift coordinate (one might, after all, have to deal with a dataset containing Doppler velocities measured in different parts of the spectrum) and between redshifts and velocities (the latter are physical quantities – derivatives of spatial positions; the former represent a spectral property that is transformed via one of several formalisms to what is commonly referred to as a Doppler velocity – when we get to relativistic situations the connection with physical velocities becomes tenuous).
We shall refer to these as the STC coordinates. The issue is that time is bound to a position and positions are time-variable. Similarly, spectral and redshift data are tied to reference frames that may or may not be time-variable.
In the past, most data archives have not been extremely concerned with dotting every i and crossing every t, in a meticulous specification of all the details of all coordinate axes. They assumed sets of coordinate system-related defaults. Such context-dependent defaults are quite acceptable: issues are generally well-defined, obvious, and clear for single-observatory observations, even when not all is explicitly specified. However, 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; this means that every little tidbit of information needs to be documented in the metadata.
Here are some simple examples of situations and questions that need an answer, when such context-dependent defaults do not work anymore:
- Are Doppler velocities optical or radio?
- What is the spatial reference position for the time stamps; if corrected for pathlength, what direction was assumed?
- Does “J2000” mean “FK5/J2000” or “ICRS”?
- At which spatial position was this observation made?
- How can we get information on Galactic background objects from solar system observations, and vice versa?
3. Requirements and Use-Context
The top-level requirement for the design of the Space-Time Coordinate metadata can be formulated in a very simple way. It is that they:
- provide sufficient and necessary information
- are self-consistent
That is really all. However, to fit it into the larger VO Data Model context, we should leverage two more requirements:
- the system should be extensible to include other coordinates and accommodate coordinate metadata descriptions that include axes other than the STC ones
- it must be possible to define relocatable frames to accommodate, for instance, simulation data
From this one can derive secondary requirements: the metadata must include complete descriptions of the following three concepts:
- the coordinate frames that anchor the STC coordinate axes, collectively referred to as the Coordinate System
- a specific position in these frames, i.e., a coordinate object, that includes not only coordinate values, but also essential attributes such as errors and resolutions
- the volume in coordinate space that is taken up by the data object that these metadata are attached to
In all, there are four different contexts for the use of the STC metadata:
- STC Resource profile – to specify the spatial, temporal, redshift, spectral coverage of a data collection which is ultimately used by clients to determine whether or not the resource is of interest for their purposes
- Query – to specify the STC constraints of a particular request
- Catalog information – clearly, the response to a catalog query may contain coordinate information (e.g., positions), but it should also contain coverage information, specifically, the intersection of the catalog’s coverage and the query’s STC constraints
- Observational data – here we need the volume in coordinate space that is occupied by the observational data, as well as the coordinates of the observatory; note that these may not necessarily be given in the same Coordinate System
We will return to the use of STC in these contexts in Section 6. As commented before, the emphasis of this document is on the structure of the STC metadata. Hence, though the classes and attributes are laid out in detail, little is said about methods.
4. Design
It may be helpful to refer to the UML diagrams in the Appendix, while reading this section.
4.1 The Metadata Components
There are three metadata components in the design, as already noted in the derived requirements:
- Coordinate
system
This object contains the coordinate frames that together define a complete coordinate system. There may be one or more per document. - Coordinate
values
This is an object that specifies a specific position in the space defined by the coordinate system it refers to. The object contains values, units, errors, resolution, size, and pixel size, though not all of these need be present. - Coordinate
areas or ranges
This is an object that defines a volume in the coordinate space specified by the coordinate system it refers to. Regions are a special case, specifically designed for 2-D (and 3-D unit-sphere) spatial coordinates.
4.2 Coordinate System (CoordSys)
The CoordSys class is nothing more than a collection of one or more Coordinate Frames, where each frame describes a set of one or more coordinate axes that belong together.
4.2.1 Coordinate Frame (CoordFrame)
A CoordFrame object provides the metadata on one or more coordinate axes. Usually, we will be dealing with one axis. The exception is the spatial frame which may control up to three position coordinate axes, as well as their time-derivatives (velocity). CoordFrame is really the base class for coordinate frames. To ensure its full generality, this base class should only contain a name. Real-life coordinate frames should be derived from this class.
4.3 Coordinates (Coords)
The Coords object specifies a particular position in coordinate space. It requires a reference to a CoordSys object and it is made up of one or more Coordinate objects, corresponding to the Coordinate Frames defined in the Coordinate System. All objects are optional. However, in order to constitute a meaningful Coords object, one would expect at least one of the Coordinate objects to be present.
4.3.1 Coordinate
A Coordinate is an aggregate of up to 6 objects: CoordName, CoordValue, CoordError, CoordResolution, CoordSize, and CoordPixSize. Of these, only CoordName is required to be present. The others, which in their simplest forms consist of a scalar numerical Value, are all optional, though one may doubt whether a Coordinate object that contains none of these has any meaning. Although at first sight it would seem that at least CoordValue is indispensable, we shall show that there are legitimate cases where the value may be absent. Coordinate has a single Unit attribute that applies to all components. Classes that are derived from Coordinate may differ in the data types of the objects, their structure and meaning, as well as restrict allowable values for Unit. [Initially we intended allowing each component to have its own Unit, the rationale being that this is the way users treat them, a practice that is often followed in catalogs; for instance, while positional coordinate values are usually expressed in degrees, their errors and sizes are commonly quoted in arcseconds.]
4.3.1.1 CoordName
This required member of Coordinate is a simple string, presumably taken from a limited enumerated list. We may want to consider using UCDs for the Name.
4.3.1.2 CoordValue
The value of the Coordinate consists of a scalar numerical or string value and a Unit string.
4.3.1.3 CoordError
Again, this consists of a scalar Value, representing an rms error. However, we expect a variety of derived classes to appear with more sophisticated error descriptions. And clearly, the multi-dimensional coordinates require more information.
4.3.1.4 CoordResolution
The resolution along the Coordinate is expressed as a FWHM Value. In cases where this is a poor description, one should derive a more sophisticated class.
4.3.1.5 CoordSize
The size along the Coordinate is also expressed as a FWHM Value, though one would expect derived classes with more precise definitions (e.g., Holmberg diameter). This object is used in three contexts: in a resource profile it indicates the typical size (say, FOV) of datasets in the resource; in queries it serves the same purpose; and in catalog datasets it allows the server to provide Coordinate positions of objects as well as their sizes.
4.3.1.6 CoordPixSize
The pixel size (scalar Value) is often a useful figure to characterize resources, queries, and datasets, even if it only provides a typical value.
4.4 Coordinate Area (CoordArea)
The Coordinate Area class defines the volume in coordinate space that is occupied by the object it is attached to. It is an aggregate of one or more ranges in individual coordinates and is required to have a reference to a CoordSys. Multiple ranges along each coordinate axis are OR-ed together; the ranges of different axes are logically AND-ed together.
4.4.1 ScalarInterval
A ScalarInterval consists of a LoLimit and/or a HiLimit, both of which contain a double value and a unit. Note that they do not both have to be present, but at least one of them needs to be. ScalarInterval has Boolean attributes LoLimInclude and HiLimInclude and a FillFactor attribute with a value between 0 and 1 that indicates what fraction of the interval is really coovered.
4.5 AstroCoordSys
The AstroCoordSys is derived from CoordSys. It still is a collection of coordinate frames, but it is required to contain specific frame classes derived from the CoordFrame base class, in addition to zero or more additional (generic) frames. The required frames are TimeFrame, SpaceFrame, and SpectralFrame; DopplerFrame is optional.
4.5.1 TimeFrame
The Time Frame contains two or three objects: a reference position, a time scale, and, optionally, a time reference direction.
4.5.1.1 ReferencePosition
The reference position, or spatial coordinate origin, may be chosen from a standard list of such origins (StdRefPosition) or specified as a particular coordinate position (CoordOrigin). In the latter case, one may nest the origins, but eventually they should be referenced to a known standard origin. The list of standard origins includes (see Table 1): GEOCENTER, BARYCENTER, HELIOCENTER, TOPOCENTER, LSR, GALACTIC_CENTER, SUPER_GALACTIC_CENTER, EMBARYCENTER, MOON, MERCURY, VENUS, MARS, JUPITER, SATURN, URANUS, NEPTUNE, PLUTO, RELOCATABLE; additional values (e.g., planetary satellites, non-planet solar system objects) are allowed, provided they are identified in a referenced ephemeris or other authoritative source (see: Section 4.5.2.4). Ultimately, one must be able to tie the coordinate origin down to the geocenter, be it through a geographic position, an IAU resolution, an orbit ephemeris, or a solar system ephemeris – unless one deals with simulation data that have a RELOCATABLE origin. Planetary Ephemeris is required for any position related to a solar system entity other than the geocenter. ReferencePosition is a standard class that is being used for all four STC frames, though there are different restrictions for the four usages. For the Time Frame LSR and RELOCATABLE are not allowed values; for simulations one may want to use a custom position at the origin of the relocatable spatial frame.
Table 1. Standard Reference Positions
|
Reference Position |
Description |
Comments |
|
GEOCENTER |
Center of the earth |
|
|
BARYCENTER |
Center of the solar system barycenter |
|
|
HELIOCENTER |
Center of the sun |
|
|
TOPOCENTER |
“Local”; in most cases this will mean: the location of the telescope |
|
|
LSR |
Local Standard of Rest: 19.5 km s-1 in the direction of FK4(B1900.0:270,+30) or GALACTIC_II(56,+23) |
Only to be used for redshifts and Doppler velocities |
|
GALACTIC_CENTER |
Center of the Galaxy |
|
|
SUPER_GALACTIC_CENTER |
Center of the Local Super Cluster |
At the center of the Virgo Cluster, i.e., M87 |
|
EMBARYCENTER |
Earth-moon barycenter |
|
|
MOON |
Center of the moon |
|
|
MERCURY |
Center of Mercury |
|
|
VENUS |
Center of Venus |
|
|
MARS |
Center of Mars |
|
|
JUPITER |
Center of Jupiter |
|
|
SATURN |
Center of Saturn |
|
|
URANUS |
Center of Uranus |
|
|
NEPTUNE |
Center of Neptune |
|
|
PLUTO |
Center of Pluto |
|
|
RELOCATABLE |
Relocatable center; for simulations |
Only to be used for spatial coordinates |
4.5.1.2 TimeScale
The time scale must be chosen from among the list recognized by the IAU (see Table 2): TT, TDT, ET, TAI, IAT, UTC, TDB, TCG, TCB, LST. TT is the default; TDT and ET are obsolete synonyms for TT; IAT is an unofficial synonym for TAI; and use of LST is to de discouraged. At some point other time scales may need to be added as they become recognized, such as planet- or moon-based time scales. For now we will also allow the value LOCAL, only to be used with RELOCATABLE space frames (see: Section 4.5.1.1). For a handy explanation of time scales, see: http://tycho.usno.navy.mil/systime.html
|
Timescale |
Description |
Comments |
|
TT |
Terrestrial Time |
|
|
TDT |
Terrestrial Dynamic Time; synonym for TT |
|
|
ET |
Ephemeris Time: predecessor of, and continuous with, TT |
|
|
TAI |
International Atomic Time; 32.184 s behind TT |
|
|
IAT |
Synonym for TAI |
|
|
UTC |
Coordinated Universal Time; 32 s behind TAI in 2004 |
Includes leap seconds |
|
TDB |
Barycentric Dynamical Time; synchronous with TT, except for variations in earth’s orbital motion |
|
|
TCG |
Geocentric Coordinate Time; properly relativistic time, running about a factor 10-10 slower than TT |
|
|
TCB |
Barycentric Coordinate Time; properly relativistic time, running about a factor 10-8 slower than TDB |
|
|
LST |
Local Siderial Time |
Ground-based observations only |
|
LOCAL |
“Local” time |
Only to be used for simulations, in conjunction with RELOCATABLE spatial coordinates |
4.5.1.3 TimeRefDirection
If the Time Frame’s Reference Position is not TOPOCENTER, times have clearly been transformed from observed time at the location where the observation was made to some other spatial location. In order to effect such a transformation, a direction of origin must have been assumed for the observed phenomenon; that direction is given in TimeRefDirection.
4.5.2 SpaceFrame
The Space Frame contains a reference position, a reference frame, and a coordinate flavor object.
4.5.2.1 ReferencePosition
This object is of the same class as described in Section 4.5.1.1, with the following restrictions. The value RELOCATABLE is allowed, but LSR is not. Obviously, if an explicit coordinate origin is provided, those coordinates should be referenced to a different CoordSys; one may still nest them, but eventually they should be referenced to a known standard origin.
4.5.2.2 CoordFlavor
This class is peculiar to spatial coordinates. It indicates the intrinsic dimensionality of the coordinate system (1, 2, or 3), and the type of coordinates: Cartesian, spherical, unitsphere, or GEOGRAPHIC (longitude, latitude, and altitude). For this last one we allow the special Units value “deg deg m”. This list will likely be expanded with, for instance, polar and cylindrical coordinates.
4.5.2.3 SpaceRefFrame
While the Reference Position pins down the origin of the spatial coordinate system, the Space Reference Frame specifies its orientation. In most cases this will be a StdFrame, consisting of a RefSystem (FK4, FK5, ECLIPTIC, ICRS, GALACTIC_I, GALACTIC_II, SUPER_GALACTIC, AZ_EL, BODY; the first three require a CoordEquinox). The Standard Frame may be BODY if the Reference Position is specified as a solar system object. In addition, we allow solar, lunar, planetary, and planetary satellite coordinate frames (planetocentric and planetographic) as defined by Sections 7.2-7.5 of the Explanatory Supplement to the Astronomical Almanac (University Science Books 1992, Ed. P. K. Seidelmann); see also Fränz & Harper (2002, Plan & Sp Sci 50, 217). See Table 3.
Alternatively, a CustomFrame may be specified by way of the pole (Z) axis and the longitude zero-point (X-axis) directions.
Table 3. Standard Reference Frames
|
Reference Frame |
Description |
Comments |
|
FK4 |
Fundamental Katalog, system 4; Besselian |
Requires Equinox; default B1950.0 |
|
FK5 |
Fundamental Katalog, system 5; Julian |
Requires Equinox; default J2000.0 |
|
ECLIPTIC |
Ecliptic coordinates |
|
|
ICRS |
International Celestial Reference System |
|
|
GALACTIC_I |
Old Galactic coordinates |
|
|
GALACTIC_II |
“New” Galactic coordinates |
|
|
SUPER_GALACTIC |
Super galactic coordinates; pole at GALACTIC_II(47,+5) |
|
|
AZ_EL |
Local azimuth and elevation |
Ground-based observatories |
|
BODY |
Generic “BODY” coordinates |
|
|
GEO |
Geographic coordinates |
|
|
GEOD |
Geodetic coordinates |
|
|
MAG |
Geomagnetic coordinates |
|
|
GSE |
Geocentric Solar Ecliptic coordinates |
|
|
GSM |
Geocentric Solar Magnetic coordinates |
|
|
SM |
Solar Magnetic coordinates |
|
|
HGC |
Heliographic coordinates |
See Explanatory Supplement, Section 7.2 |
|
HEE |
Heliocentric Earth Ecliptic coordinates |
|
|
HEEQ |
Heliocentric Earth Equatorial coordinates |
|
|
HCI |
Heliocentric Inertial coordinates |
|
|
HCD |
Heliocentric coordinates of Date |
|
|
MERCURY_C |
Planetocentric coordinates on Mercury |
See Explanatory Supplement, Section 7.4 |
|
VENUS_C |
Planetocentric coordinates on Venus |
See Explanatory Supplement, Section 7.4 |
|
LUNA_C |
Lunacentric coordinates |
See Explanatory Supplement, Section 7.3 |
|
MARS_C |
Planetocentric coordinates on Mars |
See Explanatory Supplement, Section 7.4 |
|
JUPITER_C _III |
Planetocentric coordinates on Jupiter, system III |
See Explanatory Supplement, Section 7.4 |
|
SATURN_C _III |
Planetocentric coordinates on Saturn, system III |
See Explanatory Supplement, Section 7.4 |
|
URANUS_C _III |
Planetocentric coordinates on Uranus, system III |
See Explanatory Supplement, Section 7.4 |
|
NEPTUNE_C _III |
Planetocentric coordinates on Neptune, system III |
See Explanatory Supplement, Section 7.4 |
|
PLUTO_C |
Planetocentric coordinates on Pluto |
See Explanatory Supplement, Section 7.4 |
|
MERCURY_G |
Planetographic co-rotating coordinates on Mercury |
See Explanatory Supplement, Section 7.4 |
|
VENUS_G |
Planetographic co-rotating coordinates on Venus |
See Explanatory Supplement, Section 7.4 |
|
LUNA_G |
Lunagraphic co-rotating coordinates |
See Explanatory Supplement, Section 7.3 |
|
MARS_G |
Planetographic co-rotating coordinates on Mars |
See Explanatory Supplement, Section 7.4 |
|
JUPITER_G _III |
Planetographic co-rotating coordinates on Jupiter, system III |
See Explanatory Supplement, Section 7.4 |
|
SATURN_G _III |
Planetographic co-rotating coordinates on Saturn, system III |
See Explanatory Supplement, Section 7.4 |
|
URANUS_G _III |
Planetographic co-rotating coordinates on Uranus, system III |
See Explanatory Supplement, Section 7.4 |
|
NEPTUNE_G _III |
Planetographic co-rotating coordinates on Neptune, system III |
See Explanatory Supplement, Section 7.4 |
|
PLUTO_G |
Planetographic co-rotating coordinates on Pluto |
See Explanatory Supplement, Section 7.4 |
4.5.2.4 Planetary ephemeris (if needed)
The planetary ephemeris object PlanetaryEphem indicates which solar system ephemeris was used in the AstroCoordSys. In general, this will be JPL’s DE200 or DE405 (default). It should only be present if a solar system ephemeris was used, for instance because of planet-based coordinate systems or the application of barycenter corrections. In addition, the information in Sections 7.2-7.5 of the Explanatory Supplement to the Astronomical Almanac (University Science Books 1992, Ed. P. K. Seidelmann) may be considered a known authoritative source for the purpose of spatial reference frames. If the SpaceRefFrame (see: Section 4.5.2.3) is defined on a body, ephemeris information on its coordinate system (pole, primary meridian) must be included in the PlanetaryEphem.
4.5.3 SpectralFrame
The Spectral Frame only contains a reference position.
4.5.3.1 ReferencePosition
This object is of the same class as described in Section 4.5.1.1, without any restrictions.
4.5.4 RedshiftFrame
The Redshift Frame contains two objects: a reference position and a Doppler definition object.
4.5.4.1 ReferencePosition
This object is of the same class as described in Section 4.5.1.1, without any restrictions.
4.5.4.2 DopplerDefinition
This object specifies what the definition of redshift is and how it should be translated to Doppler velocity. Allowed values are OPTICAL, RADIO, and RELATIVISTIC.
OPTICAL:
RADIO:
RELATIVISTIC:
It should be emphasized that Doppler velocities are formal velocities; i.e., defined by a formalism and not necessarily physical in nature. One should also note here that RELATIVISTIC is not strictly correct and we should, at some point, probably move toward compliance with the third FITS WCS paper.
4.6 AstroCoords
The AstroCoords object specifies a particular position in STC coordinate space. It requires a reference to an AstroCoordSys object and it is made up of 1 to 5 Coordinate objects, at most one each of the following:
- Time of class TimeCoord
- Position of class SpatialCoord
- Velocity of class SpatialCoord
- Redshift of class SpectralCoord
- Spectrum of class SpectralCoord
Note that all objects are optional. However, in order to constitute a meaningful AstroCoords object, one would expect at least one of them to be present.
The three specialized STC Coordinate classes, TimeCoord, SpatialCoord, and SpectralCoord, are all derived from a generic base-class Coordinate (see: Section 4.3.1). Instead of providing the actual coordinate elements, the object may refer to a FITS file that provides some or all of them.
4.6.1 TimeCoord
The error, resolution, size, and pixel size components of this class (derived from Coordinate) are fully generic, i.e., consisting of a scalar value (double) and unit string. The allowed time units are ‘s’ (second), ‘d’ (day = 86400 s), ‘yr’ or ‘a’ (Julian year = 365.25 d), ‘cy’ (Julian century = 36525 d).
The CoordValue object in TimeCoord is of class AstronTime.
4.6.1.1 AstronTime
A time (i.e., a particular instant in history) is an aggregate of two or three objects: a TimeScale (Section 4.5.1.2), an AbsoluteTime, and, optionally, a RelativeTime (or ElapsedTime). Note that, in principle, all times are really elapsed times. The difference is that for absolute times there is an implicit agreement as to what the reference instant in time is.
4.6.1.1.1 AbsoluteTime
AbsoluteTime contains is instantiated as one of its four subclasses:
- ISOTime: a time instant expressed in a subset of the ISO-8601 format; i.e., a string of format yyyy-mm-ddThh:mm:ss.sss…
2. JDTime: a decimal number (note: in principle unlimited precision!) representing the Julian Day of a particular instant.
3. MJDTime: a decimal number (note: in principle unlimited precision!) representing the Modified Julian Day of a particular instant (MJDTime = JDTime – 2,400,000.5).
4. TimeOrigin: a string with the value RELOCATABLE, primarily for simulation work, to be used in conjunction with TimeScale LOCAL and a RELOCATABLE SpaceFrame.
4.6.1.1.2 RelativeTime
Relative time is a decimal number representing the amount of time elapsed since AbsoluteTime.
4.6.2 SpatialCoord
Spatial coordinates are special in that they can be explicitly multi-dimensional and that they serve positions as well as their first time derivatives (physical velocities – not to be confused with redshifts or Doppler velocities). Not only does this mean that scalar values are to be replaced by vectors, but components like errors, resolutions, and sizes become more complicated. SpatialCoord is an aggregate of CoordName, Dimensionality, CoordValue (class SCValue), CoordError, CoordResolution, CoordSize, CoordPixSize (all class MultiCoord). We stress that Dimensionality should force all components of SpatialCoord to be consistent in type and content.
It may well be that there is merit in sub-classing a VelocityCoord class, since sexagesimal formats are not needed and a time unit needs to be added. Either way, we suggest that the units for velocity be the same as for position, but that a (standard) time unit be added for the denominator.
4.6.2.1 Dimensionality
A single object of this class controls all components of the SpatialCoord class by specifying how many coordinate axes need to be represented. Note that this is distinct from the CoordFlavor object described in Section 4.5.2.2: CoordFlavor specifies the number of spatial axes defined in the AstroCoordSys; Dimensionality specifies how many of those are actually provided in the AstroCoords object – one could envision a catalog with only Right Ascensions, where the CoordFlavor would be spherical, 2-dimensional, but the dimensionality just be 1.
4.6.2.2 CoordName
This object now contains a list of <Dimensionality> strings with the names of all axes present.
4.6.2.3 SCValue
The spatial coordinate value is a vector (length determined by Dimensionality) of decimal values (SValue).
4.6.2.4 MultiCoord
Expressing errors and sizes for vectors is slightly more complicated. If the errors are independent for each vector component, then a simply SValue object (vector of decimal values with a Unit) will do. If they are not independent (or if the resolution is tilted), one can either add a PosAngle to the SValue object, or specify the MultiCoord object through a Matrix of decimal values with a Unit.
4.6.3 SpectralCoord
The SpectralCoord is used for the spectral as well as the redshift components of the AstroCoords object. It is a completely generic Coordinate class (see: Section 4.3.1) where Name is a string and all other components consist of a double Value and a string Unit.
4.7 AstroCoordArea
The Coordinate Area class defines the volume in coordinate space that is occupied by (or: the coverage of):
- Resource
- Query
- Catalog data set
- Observation
It is an aggregate of one or more ranges in individual coordinates and is required to have a reference to an AstroCoordSys. Multiple ranges along each coordinate axis are OR-ed together; the ranges of different axes are logically AND-ed together.
4.7.1 TimeInterval
A TimeInterval consists of a StartTime and/or a StopTime, both of which are instantiations of AstronTime. Note that they do not both have to be present, but at least one of them needs to be. TimeInterval has Boolean attributes LoLimInclude and HiLimInclude, and attribute FillFactor.
4.7.2 SpatialInterval
The spatial position area has more options:
- Simple intervals on a per-axis basis
- Circle or sphere
- Regions
4.7.2.1 CInterval
CInterval is the vector equivalent of ScalarInterval (see: Section 4.4.1). LoLimit and HiLimit are of type SCValue (see Section 4.6.2.3).