Space-Time Coordinate Specification for VO Metadata


The following table attempts to explain the various elements in the Space-Time DTD and their use in each of the five areas of application:
  1. Resource description (especially the coverage portion)
  2. The query (search location/area)
  3. A table of catalog data
  4. The observation location part of retrieved observational data
  5. The observatory location part of retrieved observational data
R means "Required", O means "Optional", N/A means "Not Applicable".  FOV is "Field Of View".
"-->" indicates a sub-element contained in the present sub-element.
This table just prints in landscape format (at least on A4).
The DTD may be found here.
An example (Example 1 from the VOTable) is offered in a long and a short version.
In addition, there is an example of a resource description ( Chandra Data Archive) and an ObservationLocation+ObservatoryLocation (Chandra observation 770). Note that the VO region format has not yet been formalized; when done it will replace the FITS Region file. A draft may be found here.
Note also that the CoordElement is very well suited, too, to describe other data space axes, e.g., frequency/wavelength/energy.


Element
Sub-element
Attribute
Data Type
Allowed Values
Default
Resource Description
Query
Catalog Table
Data: Observation
Data: Observatory
CoordSystem

ID
ID


R
R
Observation and Observatory may be able to share the same CoordSystem, but in most cases this will not be possible


planetary_ephem
string
JPL-DE200
JPL-DE405
JPL-DE405
O
Only needed when transformations across the solar system are made, such as barycenter corrections

CoordFrame
coord_ref_frame
string
ICRS
FK5
FK4
ECLIPTIC
GALACTIC
SUPER-
  GALACTIC
AZ_EL
EARTH
SPHERICAL-
  _BODY
ICRS
O
For casual use this does not matter (if the value is ICRS, FK5 or even FK4), but for serious use it should be provided; this attribute specifies the coordinate system (equatorial, ecliptic, galactic, etc.); if AZ_EL the position of the (ground-based) observatory is needed, or course; SPHERICAL_BODY allows specification of a longitude-latitude coordinate system on a solar system body specified by CoordRefPosition
O
Should be EARTH for ground-based observatories when time is given in LST or coordinates in AZ_EL

  --> CoordEquinox

string
B1850.0
B1900.0
B1950.0
J2000.0
J2000.0
O
Part of CoordFrame
Is a concatenation of its two attributes; has only meaning for equatorial coordinates; should be generally J2000 for observations
coord_equinox_sys
 string
 B
J
 J
 O
B=Besselian; J=Julian


coord_equinox_date
 double
 1850.0
1900.0
1950.0
2000.0
 2000.0
 O
Pre-1984 dates should be combined with B and FK4 and earlier; post-1984 dates with J and FK5 or ICRS

CoordRefPosition

string
GEOCENTER
BARYCENTER
HELIOCENTER
TELESCOPE
<solar system bodies>

GEOCENTER
O
In most cases it will not matter; exceptions:
1) observations of objects in the near field (i.e., solar system) for a telescope that moves far from the earth;
2) value TELESCOPE is required for data in AZ_EL coordinate systems
3) coordinate systems on other solar system bodies
O
Will be GEOCENTER in most cases; exceptions for observatories located on other solar system bodies

TimeFrame
time_scale
string
TT TDT
ET
TDB
TCG
TCB
TAI IAT
UTC
LST
TT
R?
TT is the preferred time_scale, though UTC may be more common; it is recommended that this attribute be specified; LST requires special care: the observatory position should be properly specified; use of TCG and TCB is unlikely in the near future

  -->  TimeFormat
time_format
string
JD
MJD
ISO8601
SECOND
DAY
ISO8601
O
Part of TimeFrame
SECOND and DAY are relative times and require time_ref_value to be set; JD and MJD should not be combined with time_scale LST
time_ref_format
 string
 JD
MJD
ISO8601
 ISO8601
 O
Format  for time_ref_value; required if time_ref_value is expressed in JD or MJD


time_ref_value
dateTime
double


O
Required if time-format SECOND or DAY is specified

TimeRefPosition

string
GEOCENTER
BARYCENTER
HELIOCENTER
TELESCOPE
<solar system bodies>
GEOCENTER
O
Not important for resource description
O
O
Important for high time precision; it makes sense to combine BARYCENTER only with time_scale TDB or TCB

CoordType
coord_naxes
int
1
2
3
2
O
This indicates how many spatial coordinate axes to expect, exclusive of time, velocities, and Doppler velocity or redshift
coord_type
 string
 SPHERICAL
CARTESIAN
 SPHERICAL
 O
This indicates what coordinates to expect in the Coords element; 3D_SPHERICAL is a longitude and latitude, combined with a radius, such as RA, Dec, and distance, or geographic longitude and latitude and distance from the GEOCENTER


coord_vel
 boolean
 true
false
 false
 O
Velocities are expected to be present for all spatial coordinates (coord_naxes) when specified

  --> Doppler
string
 VELOCITY
REDSHIFT
O
Part of CoordType, indicating presence of Doppler velocity or redshift as the last coordinate; note that Doppler velocity is not the same as velocity along the radial coordinate


doppler_definition
string
OPTICAL
RADIO
RELATIVISTIC
OPTICAL
O
Doppler velocity is not (necessarily) a physical velocity, but a formalism to express a red- or blueshift as something like a radial velocity; most, but not all, Doppler velocities use the optical convention (delta-lambda/lambda0); if one sees LSR it's more likely to be the radio convention (delta-nu/nu0)


doppler_reference
string
OBSERVATORY
GEOCENTER
HELIOCENTER
BARYCENTER
LSR
GALACTIC-
  _CENTER
SUPER-
  _GALACTIC-
  _CENTER		BARYCENTER
O
These references attempt to correct the measured Doppler velocity for the observer's motion with respect to these various standards of rest; and they are just that: an agreed upon standard of rest; hence they should not be foolishly used, such as supergalactic Doppler velocities for Galactic objects
Coords

ID
ID


O
This element is only needed when the area or coverage is defined by CircleOrSphere
R


coord_system_id
IDREF


R

CoordFile

FITS


O
The columns in the FITS CoordFile will be expected to correspond to CoordType and Time needs to be included
O
Most likely use is here as an orbit ephemeris file

Time


is a CoordElement

O
May be omitted if not relevant
O
Required if CoordFile not used


CoordLongitude


is a CoordElement

O
First spatial coordinate for coord_type SPHERICAL

CoordLatitude


is a CoordElement

O
 Second spatial coordinate for coord_type SPHERICAL; only for coord_naxes>1

CoordR


is a CoordElement

O
 Third spatial coordinate for coord_type SPHERICAL; only for coord_naxes=3

CoordX


is a CoordElement

O
 First spatial coordinate for coord_type CARTESIAN

CoordY


is a CoordElement

O
 Second spatial coordinate for coord_type CARTESIAN; only for coord_naxes>1

CoordZ


is a CoordElement

 O
Third spatial coordinate for coord_type CARTESIAN; only for coord_naxes=3

CoordVLong


is a CoordElement

O
First velocity coordinate for coord_type SPHERICAL (RA proper motion)


cos_lat_corr
boolean
true
false
true
O
Indicates whether the cos(latitude) correction has been applied to this velocity component

CoordVLat


is a CoordElement

O
Second velocity coordinate for coord_type SPHERICAL (Dec proper motion); only for coord_naxes>1

CoordVR


is a CoordElement

O
Third velocity coordinate for coord_type SPHERICAL (radial velocity); only for coord_naxes=3

CoordVX


is a CoordElement

O
 First velocity coordinate for coord_type CARTESIAN

CoordVY


is a CoordElement

O
 Second velocity coordinate for coord_type CARTESIAN; only for coord_naxes>1

CoordVZ


is a CoordElement

O
 Third velocity coordinate for coord_type CARTESIAN; only for coord_naxes=3

CoordDoppler
Velocity


is a CoordElement

O
Doppler velocity or redshift coordinate, if Doppler has a value
CoordElement
CoordValue

double
dateTime
IDREF


R
The elements that are contained in CoordElement may provide either values directly or by reference to a FIELD or PARAM element in the document; this applies to CoordValue, CoordError, CoordResolution, CoordSize; CoordUnit should be omitted if all elements are references

CoordError

double
IDREF


O
Typical absolute uncertainty (1 sigma) in this coordinate for data in this resource
O
Maximum absolute uncertainty permitted
 O
Absolute uncertainty (1 sigma) in CoordValue


position_angle
double
0-180 degrees

O
Only allowed on second spatial coordinate: if position_angle is present, it is assumed that the errors in the first and second spatial coordinates are correlated, in which case the errors are interpreted as semi-axes of an ellipse and position_angle is the position angle from North (positive Y), CCW, of the axis corresponding to the error in the second spatial coordinate; the unit is degrees

CoordResolution

double
IDREF


O
Typical resolution (FWHM) in this coordinate for data in this resource
O
Query is for  information with resolution in this coordinate that is no worse than CoordResolution
 N/A
 O
Resolution (FWHM) in this coordinate

 N/A


position_angle
double
0-180 degrees

O
As position_angle for CoordError

CoordSize

double
IDREF


O
Typical FOV size in this coordinate for image data in this resource
O
For image data queries: minimum FOV size in this coordinate

 O
Measured object size (FWHM) in this coordinate, usually assuming an elliptical Gaussian
 N/A


position_angle
double
0-180 degrees

O
As position_angle for CoordError

CoordUnit

string


O
Units of the actual values transmitted through subelements of in a given CoordElement; does not apply to subelements that reference a FIELD or PARAM; default units: second, meter, degree and combinations thereof; days for JD and MJD formats; note that the units for value, error, resolution, and size need to be the same
CoordArea





R
Resource coverage:
This element describes the coverage of the resource; the meaning is that OUTSIDE this area there is a guarantee that NO DATA will be found
R
Search area:
This element describes the area over which the resource is to be searched for data

 O
Catalog coverage:
When catalog data are returned, this element may indicate the area over which the data are complete

 R
FOV:
This element describes the FOV of the data returned

 N/A


ID
 ID

R
A CoordArea will consist of a TimeInterval plus:
       a RegionFile
  or: a CoordInterval for each spatial coordinate
  or: a CircleOrSphere element for the spatial coordinates
Intervals for (Doppler) velocities may optionally be added


coord_system_id
IDREF


R
N/A

TimeInterval




R
Specify the Time part of the Space-Time CoordArea
N/A


start_time
double
string


O
Start time of interval
N/A


stop_time
double
string


O
Stop time of interval
N/A


start_include
boolean
true
false
true
O
Include the start time?
N/A


stop_include
boolean
true
false
true
O
Include the stop time?
N/A

RegionFile

FITS


O
The spatial part of the CoordArea is defined in a FITS Region File
N/A

CoordInterval




O
Specify the CoordArea interval for a spatial coordinate
N/A


coord_index
int
1-9

O
Coordinate index, with the first spatial coordinate being 1
N/A


lo_limit
double


O
Lower limit of interval
N/A


hi_limit
double


O
Upper limit of interval
N/A


lo_include
boolean
true
false
false
O
Include lower limit?
N/A


hi_include
boolean
true
false
true
O
Include upper limit?
N/A

CircleOrSphere




O
Specify the spatial part of a CoordArea as a circle (2D) or sphere (3D)
N/A


coords_id
IDREF


R
The referenced Coord element specifies the center of the circle or sphere
N/A


radius
double


R
Radius of circle or sphere
N/A


radius_unit
double


R
Units if the circle or sphere radius
N/A