Space-Time Coordinate (STC) Metadata Specification for VO

Version 1.30

Arnold Rots, SAO/CXC, 2012-05-30

This directory contains version 1.3 of the STC schema which is the official IVOA Recommendation version.

STC has been developed as part of the National Virtual Observatory (NVO) project, in the wider context of the International Virtual Observatory Alliance (IVOA).


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.


Standards Document

The final version of the Recommendation in
HTML or PDF format.

A chapter on STC may also be found in the NVO Book which is accessible on-line.


The STC Model and its hierarchy are presented in a PDF file, serving as an explanatory supplement and glossary to the STC Recommendation. It shows all components that are, or may be, needed to specify STC metadata in their location in the hierarchy, with brief descriptions where needed. I would suggest that one explore the model through the PDF Bookmarks.

A set of UML diagrams of the coordinate system design is provided here in PDF format.

XML Implementation

The full STC schema is documented here. Rob Seaman asked for this, but beware - it's very large.

A full list of units strings that are allowed in STC and a list of global elements.


The most significant changes from version 1.21:

STC-X: XML Schema

The XML schema implementation consists of one XML Schema file:


The XMLSpy-generated code is available in tar files for:

XML Examples

Below are 19 examples of XML files built with the schema (note that the style sheet is not yet up to date, although there is a new style sheet for coverage descriptions):

STC Region Encoding

The STC standard provides for the general exchange of Region descriptions: a simple XML document that contains either a single Region (using an element with xsi:type="STCRegion") or a list of Regions (using an element with xsi:type="STCRegionList") This is the exchange format used in the JHU footprint service. Considering that there is a standard XML serialization of Regions, it makes sense to use this format when exchanging Region descriptions in XML. An example of such a description may be found

As to the question what is the best way to incorporate Region descriptions in a larger document, I will give examples of three possible approaches. For convenience, they are all based on the little utility schema mentioned above, which allows putting all kinds of coordinate information in a simple table. This simple schema defines an STC metadata element and a plain table where components of the STC metadata can refer to specific columns in the table through ID/IDREF pairs; this ensures that the place of each datum in the metadata model is fully specified. The three examples implement the description of a 0.4x0.2 degree area in different locations on the sky, with different position angles. The table contains fiducial URLs to the supposed image data that should be ignored.

STC-S: String Implementation

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.
STC-S at the IVOA site (previous version).

Current version of the Working Draft: