1. NVOSS 2008 Santa Fe1 Space Time Coordinates Gretchen Greene (many thanks to Arnold Rots) T HE US N ATIONAL V IRTUAL O BSERVATORY Sept 2008

2. NVOSS 2008 Santa Fe2 Data Modeling STC is a recommended Data Model for the description of spatial and temporal astronomical observations –XML Standard Schema http://www.ivoa.net/xml/STC/stc-v1.30.xsd NVO Book, Section 3: Technologies and Protocols –Space-Time Coordinate Metadata for the Virtual Observatory IVOA Data Modeling Working Group http://www.ivoa.net/cgi-bin/twiki/bin/view/IVOA/IvoaDataModel Sept 2008

3. STC Metadata Description of 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. Spatial component includes: –Coordinates: Spherical, 2-D, and 3-D as well as one dimensional Spatial time derivatives: –Velocities including proper motions –Redshifts Example: for an image, the STC describes very precisely the piece of space represented or occupied by the image.. Sept 2008NVOSS 2008 Santa Fe3

4. Objective of STC Describe positions or volume in coordinate space that is: –Complete –Consistent Intertwined coordinate axes: –Time –Space (position & velocity) –Spectrum –Redshift (includes Doppler velocity) Sept 2008NVOSS 2008 Santa Fe4

5. Accuracy for Archival Research The STC data model is designed such that spatial and temporal data are encapsulated in single metadata object –Preservation of information and insurance that the future usage of observations may be correlated in space and time Archival research can determine precisely the spatial observation time periods or variants Highly accurate specification which provides exact descriptions Sept 2008NVOSS 2008 Santa Fe5

6. STC Model STC metadata comes in four contexts: – STCResourceProfile – SearchLocation – CatalogEntryLocation – ObsDataLocation Three components: – CoordinateSystem – CoordinateArea – Coordinates Sept 2008NVOSS 2008 Santa Fe6

7. Coordinate System Contains any number of Frames, of any kind AstroCoordSystem contains a specific set of frames plus optional generic ones Similarly with Coords and CoordIntervals Sept 2008NVOSS 2008 Santa Fe7

8. Coordinate Area defines a Volume Sept 2008NVOSS 2008 Santa Fe8

9. Coordinates Coordinates contain 6 components with common unit: Name (UCD?) Value Error Resolution Size Sampling size (pixel size) All may be Value (scalar or vector numeric; string) IDREF (values provided elsewhere, such as table column) Sept 2008NVOSS 2008 Santa Fe9

10. Compound Coordinates Sept 2008NVOSS 2008 Santa Fe10

11. XML Implementation Referencing mechanisms Defined an STC basetype from which many types are derived that contains the following attributes: xlink:type => We only use the value simple xlink:href => The URI where an element of the correct type is to be found idref => A reference to an existing element in the document id => An ID allowing the element to be referenced ucd => An optional UCD string Consequently, the contents of an element may be specified in three ways: 1. Through the actual contents in its body 2. Through a reference to another element in the document (idref) 3. Through a reference to an element defined elsewhere (Xlink) Sept 2008NVOSS 2008 Santa Fe11

12. Current STC VO Implementations Footprints – Description of Spatial Regions –Two forms: => element => VOTable uType attribute which associates data model with VOTable fields VOEvent - qualifying the Spatial and temporal description of a transient Event – Registries – for defining the Coverage of a Service Resource – Sept 2008NVOSS 2008 Santa Fe12

13. VOEvent 2004-07-15T08:23:56 2 148.88821 69.06529 0.03 Sept 2008NVOSS 2008 Santa Fe13

14. Footprint STCRegion 0.031645908203478207 0.0016196316956034744 -0.771999390196343 0.57924028659774 -0.261720522540062 0 Sept 2008NVOSS 2008 Santa Fe14

15. Registry Coverage A description of how a resource's contents or behavior maps to the sky, to time, and to frequency space, including coverage and resolution. The STC description of the location of the resource's data (or behavior on data) on the sky, in ti me, and in frequency space, including resolution. In general, this description should be approximate; a more precise description can be provided by the footprint service. Sept 2008NVOSS 2008 Santa Fe15

16. Where we are going with STC CfA is currently working on STC library in JAVA While the STC data model is complex to include a complete and accurate description, implementation only requires subset, e.g. single context STC data models will be used in conjunction with other IVOA data models for detailing full archives Sept 2008NVOSS 2008 Santa Fe16