STC version 2.0

Goal

Defines coordinates, coordinate related metadata and their relationships for use in the IVOA data models.

Coordinate: location within the coordinate space.

Coordinate frame: describes the coordinate space for a domain;

Coordinate system: collection of coordinate frames covering the domain space

Why STC-2.0?

Version 1 of STC was developed in 2007, prior to the development and adoption of vo-dml modeling practices. As we progress to the development of vo-dml compliant component models, it is necessary to revisit those models which define core content. Additionally, the scope of the STC-1.0 model is very broad, making a complete implementation and development of validators, very difficult. As such it may be prudent to break the content of STC-1.0 into component models itself, which as a group, cover the scope of the original.

This effort will start from first principles with respect to defining a specific project use-case, from which requirements will be drawn, satisfied by the model, and implemented in the use-case. We will make use of the original model to ensure that the coverage of concepts is complete and that the models will be compatible. However, the form and structure may be quite different. This model will use vo-dml modeling practices, and model elements may be structured differently to more efficiently represent the concepts.

Participants

domain experts: ArnoldRots

data modeler: MarkCresitelloDittmar

contributors: ArnoldRots (editor), GerardLemson, OmarLaurino, MarkCresitelloDittmar

Uses cases

1) The primary use case for this work is in support of the CubesDM. This will involve the definition of coordinates and associated metadata in several physical domains and pixel domain.

Requirements

• Structure
  • The model shall be vo-dml compliant, producing a validated vo-dml XML description.
  • shall produce documentation in standard pdf format, and as vo-dml HTML
  • shall re-use, or refer to, dependent models.
• Scope
  • Requirements of the Cube model incorporate the following concepts/relations within the STC area of discourse
    • Coordinate frames; describing domain axes, orientation and origin
    • Coordinate systems; collection of coordinate frames providing a complete description of the domain space
    • Coordinates; locations within the coodinate space, with association to corresponding domain spec.
    • Defining relation between two coordinate frames within the same domain.
    • Derived coordinates; measured or calculated values including associated errors
  • Requirements of Cube model include the Pixel, Spatial, Temporal, Spectral, and Polarization domains.
• Application/Usage
  • Users should be able to identify and use basic content with minimal specialized information.
    • in other words, a generic utility should be able to find and use core elements without knowing a lot about the various extensions and uses of those elements.
• Content
  • Shall facilitate the specification of the various concepts in all supported domains
  • Coordinate frames:
    • Shall faciliate the specification of the nature of the domain.
      • dimensionality
      • origin
      • orientation vectors ( for greater than 1D domains )
    • May be specified using generally accepted standard definitions
    • Must be customizable to facilitate origin relocation
  • Coordinate systems:
    • Shall provide the Frame specifications for the entire domain space
    • Shall NOT require specification in all supported domains
    • Any specific domain Shall NOT appear more than once in a single coordinate system specification.
  • Coordinates:
    • Shall identify a location within the coordinate domain space
    • Shall be associated with a corresponding coordinate frame or axis
    • Shall be complete value quantities, including value and units as appropriate
  • Derived Coordinates:
    • Shall relate a coordinate value with associated errors
    • Shall support multiple error associations per value to provide errors from different sources
    • Any specific error source may appear only once.
    • errors may not be separable for greater than 1-D domains ( ie: may apply to coordinate pair or triplet as a whole )
    • values associated with different domains may have correlated errors (ie: components of coordinate tuple may refer to different domains, and have non-separable errors)
  • Shall facilitate the relation of two coordinate frames through a mathematical formula (Transforms)
  • Transforms:
    • Must support the following industry standard transform specifications
      • FITS WCS
      • Linear
      • Matrix
      • Lookup

Documents

Latest Document:

The current documentation may be found on Volute:

Volute:

The current working draft of the document, including all images and source document can be found in the volute repository.. here

UML Model:

We also provide an export of the UML specification in XMI format (version 2.4.1), which is compatible with the vo-dml xslt scripts for generating the vo-dml XML representation.

vo-dml:

VO-DML XML serialization of the model and corresponding HTML page are here

Shortcuts

-- ArnoldRots - 2017-11-07

In order to provide easy access to the model elements, I propose a set of proxy components that through a simplified notation specify define specific model instances. This consistes of an AstroCoordSystem object and a set of Coordinate Value data types.

AstroCoordSystem's attributes specify a set of instances of the Temporal, Spatial, Spectral, Redshift, and/or Polarization frames. The Coordinate Value data types' attributes provide the information that defines the necessary Coordinate, Axis, and Space element instances. Note that these shortcut proxies only apply to time, space, spectrum, redshift, and polarization; Generic and Pixel coordinates need to to specified through the full model. Similarly, only standard frames and reference positions are included.

AstroCoordSystem

AstroCoordSystem's attributes specify a set of instances of the Temporal, Spatial, Spectral, Redshift, and/or Polarization frames. Note that this shortcut proxy only applies to time, space, spectrum, redshift, and polarization; Generic and Pixel coordinate frames need to to specified through the full model. Similarly, only standard frames and reference positions are included. All attributes have multiplicity 0..1. Also note that all frames have to share a common sptial reference position; this will ensure consistency between the domains.

Attribute	Data Type	Description
spaceRefFrame	StdRefFrame	Spatial reference frame: the orientation of the reference frame in space; only standard reference frames are supported
refPosition	StdRefPos	Reference position common: the origin for spatial coordinates and the spatial reference position for time, spectral, and redshift coordinates; this may be a state vector
epoch	Epoch	Optional epoch of spatial coordinate values (e.g., B1950.0, J2000.0, J2017.8345)
timeScale	TimeScale	Time scale of the TimeStamps (TT, TDT, ET, TAI, IAT, UTC, GPS, TDB, TEB, TCG, TCB, LST)
planetaryEphem	string	Planetary ephemeris used (if any), especially for spatial and temporal coordinates, but potentially also spectral and redshift coordinate values; default: DE405
timeOffset	TimeStamp	Time origin for temporal coordinate values; only required if those values are provided as TimeOffsets

Coordinate Values

There are five shortcut coordinate value data types. These put specific constraints on the CoordAxis and CoordSpace objects that they reference in the model. Note that these shortcut proxies only apply to time, space, spectrum, redshift, and polarization; Generic and Pixel coordinates need to to specified through the full model.

Time Coordinate Value

The general time value Data Type is TimeStampValue (abstract)
Derived Data Types:

• JDValue (real)
• MJDValue (real)
• timeOffsetValue (RealQuantity)
• ISOtimeValue (datetime)

These data types hold the value of the cval attribute in the corresponding data types from the model.

Spectral Coordinate Value

The general spectral value Data Type is SpectralCoord (abstract)
Derived Data Types:

• energyValue
• frequencyValue
• wavelengthValue (attribute: refractInde; default=0.0)

These data types hold the value of the cval attribute in the corresponding data types from the model.

Redshift Coordinate Value

The general redshift value Data Type is Redshift (abstract)
Derived Data Types:

• zValue or redshiftValue
• dopVelOptValue
• dopVelRadValue
• dopVelRelValue

These data types hold the value of the cval attribute in the corresponding data types from the model. The three dopvel flavors reflect the value of the dopplerDefinition attribute.

Polarization Coordinate Value

The general polarization value Data Type is Polarization (abstract)
Derived Data Types:

• polStokesValue (I, Q, U, V)
• polCircularValue (RR, LL, RL, LR)
• polLinearValue (XX, YY, XY, YX)
• polVectorValue (I, P, pa)

These data types hold the value of the cval attribute in the corresponding data types from the model.

Spatial Coordinate Value

Discussion Topics

Significant discussion threads from dm working group mailing list:

STC2 and VO-DML compliance:

Discussion on conflicts between stc2 model and vo-dml rules, specifically regarding the multiplicity of attributes.

• initial discussion in Cube thread
• STC2 Model and VO-DML Issues - spawn to independent thread
• STC2 Model and VO-DML Issues - pickup Dec. entries

Implementations