NVOSS 2008 Santa Fe1 Space Time Coordinates Gretchen Greene (many thanks to Arnold Rots) T HE US N...

NVOSS 2008 Santa Fe 1

Space Time Coordinates

Gretchen Greene(many thanks to Arnold Rots)

THE US NATIONAL VIRTUAL OBSERVATORY

Sept 2008

NVOSS 2008 Santa Fe 2

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 Grouphttp://www.ivoa.net/cgi-bin/twiki/bin/view/IVOA/IvoaDataModel

Sept 2008

http://www.ivoa.net/xml/STC/stc-v1.30.xsd

http://www.ivoa.net/cgi-bin/twiki/bin/view/IVOA/IvoaDataModel

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 Fe 3

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)


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


STC Model

STC metadata comes in four contexts:– STCResourceProfile

– SearchLocation– CatalogEntryLocation– ObsDataLocation

Three components:– CoordinateSystem

– CoordinateArea– Coordinates


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


Coordinate Area defines a Volume


Coordinates

Coordinates contain 6 components withcommon 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)


Compound Coordinates


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)


Current STC VO Implementations

• Footprints – Description of Spatial ‘Regions”– Two forms:

=> <STCRegion> element=> VOTable uType attribute which associates data model with VOTable fields

• VOEvent - qualifying the Spatial and temporal description of a transient Event– <ObsDataLocation >

• Registries – for defining the Coverage of a Service Resource– <STCResourceProfile>


VOEvent

<WhereWhen> <ObsDataLocation xmlns="http://www.ivoa.net/xml/STC/stc-v1.30.xsd">

<ObservatoryLocation/> <ObservationLocation/>

</ObsDataLocation> </WhereWhen>

<ObservationLocation> <AstroCoordSystem id="UTC-FK5-GEO" xlink:type="simple“ xlink:href="ivo://STClib/CoordSys#UTC-FK5-GEO"/> <AstroCoords coord_system_id="UTC-FK5-GEO"> <Time unit="s“><TimeInstant> <ISOTime>2004-07-15T08:23:56</ISOTime>

</TimeInstant> <Error>2</Error> </Time> <Position2D unit="deg"> <Value2> <C1>148.88821</C1> <C2>69.06529</C2> </Value2> <Error2Radius>0.03</Error2Radius> </Position2D> </AstroCoords>

</ObservationLocation>


Footprint STCRegion

<?xml version="1.0" encoding="utf-8"?> <STCRegion xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://nvo.stsci.edu/footprint"> <AstroCoordSystem id="FK5-UTC-TOPO" d2p1:href="ivo://STClib/CoordSys#FK5- UTC-TOPO" xmlns:d2p1="http://www.w3.org/1999/xlink" xmlns="http://www.ivoa.net/xml/STC/stc-v1.30.xsd" /><Union fill_factor="0.99" coord_system_id="FK5-UTC-TOPO" xmlns="http://www.ivoa.net/xml/STC/stc-v1.30.xsd">

<Area linearAreaUnit="deg" validArea="true">0.031645908203478207</Area> <Convex>

<Area linearAreaUnit="deg" validArea="true">0.0016196316956034744</Area> <Halfspace>

<Vector> <C1>-0.771999390196343</C1> <C2>0.57924028659774</C2> <C3>-0.261720522540062</C3>

</Vector> <Offset>0</Offset> </Halfspace>


Registry Coverage<xs:complexType name="Coverage"> <xs:annotation> <xs:documentation> A description of how a resource's contents or behavior maps to the sky, to time, and to

frequency space, including coverage and resolution. </xs:documentation> </xs:annotation>

<xs:sequence>

<xs:element ref="stc:STCResourceProfile" minOccurs="0"> <xs:annotation> <xs:documentation> The STC description of the location of the resource's data (or behavior on data) on the sky,

in time, and in frequency space, including resolution.

</xs:documentation> <xs:documentation> In general, this description should be approximate; a more precise description can be provided by the footprint service. </xs:documentation> </xs:annotation> </xs:element>


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


NVOSS 2008 Santa Fe1 Space Time Coordinates Gretchen Greene (many thanks to Arnold Rots) T HE US N...

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