VOResource records can contain coverage information expressed
according to the IOVA data model for Space-Time Coordinates
std:STC. The complete data model is far too large and
complex for a relational representation. However, a rough indication of
the regions in time, space, and the electromagnetic spectrum covered by
a resource is sufficient for many registry purposes, in particular
focusing queries to a smaller subset of resources by weeding out ones
which do not intersect with the region of interest.
This section defines a set of tables giving, for each resource and
STC axis, zero or more intervals covered by the data.
To make queries feasible, the STC coverage tables have fixed STC
metadata, viz,
- spatial: equatorial ra and dec, reference frame ICRS.
Units are degrees
- temporal: MJD values (i.e., days) in Terrestial Time
- spectral: wavelength in meter
- redshift: STC's OPTICAL doppler definition (i.e., Δλ/λ)
While it is expected to rarely matter at the level of precision the
registry aims for, the reference position SHOULD be the solar system
barycenter, except for time, which SHOULD be given for the geocenter.
Resource registry authors SHOULD provide their metadata in these
standard forms, but registry ingestors SHOULD make a best effort to
transform STC metadata given in other forms.
The utypes used here are taken from the STC data model, formed
according to the rules in [STCVOT].
Services providing the following tables should declare support for
the data model ivo://ivoa.net/std/RegTAP/stc in
their TAP capabilities:
Table
Utype | Description |
|---|
rr.stc_redshift
stc:AstroCoordArea.SpectralInterval | The redshift coverage of resources, given as one or more intervals;
the total coverage is (a subset of) the union of all intervals given
for a resource. Redshifts here are understood as delta lambda/lamda. |
rr.stc_spatial
stc:AstroCoordArea.Region | The spatial coverage of resources; the total coverage is (a subset
of) the union of all regions given for a resource. All coordinates are
in ICRS. There is no notion of how tight the given regions are. |
rr.stc_spectral
stc:AstroCoordArea.SpectralInterval | The spectral coverage of resources, given as one or more intervals;
the total coverage is (a subset of) the union of all intervals given
for a resource. All spectral values are understood as given as
wavelength in meters; they are supposed to express wavelength at the
barycenter of the solar system (which means instrument values should
be close enough to work). |
rr.stc_temporal
stc:AstroCoordArea.TimeInterval | The temporal coverage of resources, given as one or more intervals;
the total coverage is (a subset of) the union of all intervals given
for a resource. All times are understood as MJD in the TT time scale. |
In the following table descriptions, names and utypes of tables and
columns are normative and MUST be used as given, and all-lowercase.
Descriptions are not normative. Registry operators MAY provide
additional columns in their tables, but they MUST provide all columns
given in this specification. All values of the ivoid
column must be lowercased on ingestion.
The stc_spatial table links IVORNs with zero or more
bounding boxes, understood as intervals in Right Ascension and and
Declination (note that this means the bounding lines in latitude are
not great circles). This is an admittedly rough model, but since in
principle, a service can define arbitrarily many such patches, any
desired accuracy can be obtained. However, since the these items are
given for discovery purposes, to weed out irrelevant
services in all-VO query, and similar tasks, a fairly sketchy
specification of coverage should be sufficient.
While ADQL would support defining and querying more sophisticated
regions, using these for coverage is not straightforward; one reason is
that the natural candidate for a coverage column type, POLYGON, cannot
easily represent the entire sky or similarly large coverages, which
means ingestors would have to employ fairly advanced spherical geometry
to split such large regions up. Also, in data discovery the actual
shape of the covered regions is probably not usually important.
Currently, fully general STC specifications are allowed in registry
records. It is not expected that many registries will sensibly support
a significant part of the constructs possible and the transformations
involved. For interoperability, resource record authors should give
spherical position intervals in ICRS exclusively. A short IVOA note
will discuss the recommended format of coverage declaration.
Name
Utype | Type | Description |
|---|
ivoid
vr:Resource.identifier | adql:VARCHAR(*) | The parent resource |
ra_min
stc:AstroCoordArea.Position2VecInterval.LoLimit2Vec.C1 | adql:REAL | A left edge of a bounding box on the sky the resource covers. |
ra_max
stc:AstroCoordArea.Position2VecInterval.HiLimit2Vec.C1 | adql:REAL | An right edge of a bounding box on the sky the resource covers. |
de_min
stc:AstroCoordArea.Position2VecInterval.LoLimit2Vec.C2 | adql:REAL | A lower edge of a bounding box on the sky the resource covers. |
de_max
stc:AstroCoordArea.Position2VecInterval.HiLimit2Vec.C2 | adql:REAL | An upper edge of a bounding box on the sky the resource covers. |
The ivoid column should be an explicit foreign key into
resource.
It is recommended to maintain an index on
the area_covered column.
Temporal coverage is given by associating one or more time intervals
to an IVORN. Both start and end are mandatory, i.e., there are no
half-open intervals. All times are given in days in the MJD epoch.
While it should rarely matter, we note that times are supposed to be in
TT in the geocenter.
Name
Utype | Type | Description |
|---|
ivoid
vr:Resource.identifier | adql:VARCHAR(*) | The parent resource |
time_start
stc:AstroCoordArea.TimeInterval.StartTime | adql:REAL | Lower limit of a time interval covered by the resource. |
time_end
stc:AstroCoordArea.TimeInterval.EndTime | adql:REAL | Upper limit of a time interval covered by the resource. |
The ivoid column should be an explicit foreign key into
resource.
It is recommended to maintain indexes on
both the time_start and time_end columns.
The stc_spectral table is constructed analoguous to
stc_temporal, i.e., each resource has one or more
associated intervals, for which both upper and lower limit is mandatory.
The limits are given as wavelengths in meters, for the reference
position in the barycenter of the solar system.
Name
Utype | Type | Description |
|---|
ivoid
vr:Resource.identifier | adql:VARCHAR(*) | The parent resource |
spectral_start
stc:AstroCoordArea.SpectralInterval.LoLimit | adql:REAL | Lower limit of a spectral interval covered by the resource. |
spectral_end
stc:AstroCoordArea.SpectralInterval.HiLimit | adql:REAL | Upper limit of a spectral interval covered by the resource. |
The ivoid column should be an explicit foreign key into
resource.
It is recommended to maintain indexes on
both the spectral_start and spectral_end columns.
As stc_temporal and stc_spectral, the
stc_redshift table associates resources with intervals of
values, in this case with redshifts, given as Δλ/λ (the STC data model's
OPTICAL definition).
Name
Utype | Type | Description |
|---|
ivoid
vr:Resource.identifier | adql:VARCHAR(*) | The parent resource |
redshift_start
stc:AstroCoordArea.RedshiftInterval.LoLimit | adql:REAL | Lower limit of a redshift interval covered by the resource. |
redshift_end
stc:AstroCoordArea.RedshiftInterval.LoLimit | adql:REAL | Upper limit of a redshift interval covered by the resource. |
The ivoid column should be an explicit foreign key into
resource.
It is recommended to maintain indexes on
both the time_start and time_end columns.
