Space-Time Coordinate (STC) Metadata Specification for VO

Arnold Rots, SAO/CXC

Scope

The objective is to provide a metadata description of the volume in space-time parameter space that is occupied by, available in, or requested by: a data set of any kind, a resource, or a query. The "space" part of this parameter space includes spatial coordinates of any kind: spherical coordinates, 2-D (e.g., detector coordinates) and 3-D Cartesian coordinates, one-dimensional coordinates. Also included are the spatial time derivatives: velocities (space velocities and proper motions), spectral coordinates, and redshifts/Doppler velocities. These last ones are treated separately since they are derived quantities based on a formalism, rather than physical velocities (i.e., the value depends on the formalism, which is not applicable to true velocities).

What this means for an image, for instance, is that the metadata describes very precisely and unambiguously what piece of space is represented or occupied by the image. However, a separate metadata object is still needed to specify how that spatial volume is projected onto a pixel array. After careful consideration it was decided that separating the information into two metadata objects (one that specifies the space-time coordinates, including the pixel space, associated with the data and another that specifies the projection onto that pixel array) is the correct way to model the metadata in this area.

We strongly emphasize that space and time metadata need to be encapsulated in a single metadata object. Although it is true that for the majority of the data that are moved around this is totally unimportant (very few people besides historians will care when a particular photograph of M81 was taken), there are a number of cases where it is crucially important (e.g., high time resolution pulsar observations). We feel that the link needs to be enforced from the outset; it will be very difficult to retrofit it later if it were initially neglected. In other words: we need to do this right from the beginning.

Documentation

The full design is presented in a Proposed Recommendation of the Data Modeling Working group of the International Virtual Observatory Alliance (IVOA).

STC-X: XML Schemata

The XML Schema implementation is also posted on the IVOA website and consists of three XML Schema files:

XML Examples

Below are five examples of XML files built with the three schemata:

STC-S: String Implementation (Linear STC)

STC-S is a "command line" implementation of STC, meant to provide a concise and transparent encoding of the STC metadata suitable for embedding in, for instance, an ADQL query or a Dublin Core registry description.