Unified Content Descriptors Semantic types for astronomy

Unified Content DescriptorsSemantic types for astronomy Roy Williams, California Institute of Technology

History • Created at CDS Strasbourg • (the first VO prototype) • Harvested • From 5000 tables, 20000 table columns • To create ~450 UCD words • Example • pos.eq.ra means right ascension • think of a UCD as a “semantic type”: int n; float x; pos.eq.raalpha;

UCD is a semantic type (not a name) example: tablae of double stars name alpha1 delta1 alpha2 delta2 ucd pos.eq.ra pos.eq.dec pos.eq.ra pos.eq.dec

Summary • UCD is inherently fuzzy • does not contain all metadata • UCD is a description, not a unique name • UCD already works • easy to shoot at! • UCD is mined from large amount of metadata • phenomenological, not opinion NaturalLanguage DataModel UCD Thinner, Exact Richer, Fuzzier

Why is UCD useful • Finding table attributes • Aladin can draw based on UCD info • Cone search demands UCD labelling of output • Searching a registry that has tables • Relevance of a table based on its UCDs • Registry metadata for some data types defined with UCDs • VOResource includes VOTable, which has UCD description of table attributes. • Do this with a Matching Function • Which UCD matches best the UCD I am looking for? • Data exchange from legacy systems

UCD and utype • UCD is a type, struct Myposition { pos.eq.ra alpha; ....} • utype is a data-model-part struct VOStandardPosition { float RA; ...} standard meaning standard meaning

General Syntax of IVOA UCD • [namespace:] w.o.r.d; [namespace:] w.o.r.d; .... • Character set is • A-Z, a-z, 0-9, hyphen, + 3 special • No white space • Case insensitive • Three special characters . period for hierarchy : colon for namespaces ; semicolon for separating words

UCD Tree

Multiple words First word: what semantic type is this thing? -- other words are helpers

Names need a Namespaces “We took the table and chair dimensions, and wrote them in a table.” Namespace = furniture Namespace = wordProcessing FITS keywords have no namespace! What does “TARGET” mean?

Namespaces • Namespaces • Introduce special character colon : • example:mynamespace:em.opt.myfilter • Default namespace is ivoa • Should be an attempt to integrate to core • UCD committee • Example: SIAP 1.0 has VOX namespace • VOX:Image_AccessReference (should be meta.ref) • VOX:WCS_CoordRefPixel (should be pos.wcs.crpix)

Cone Search • service request is a cone • service response is a table: blahblah foo bar banana syzygy meta.id pos.eq.ra pos.eq.dec Must have 3 columns with these UCDs May be other columns

UCD Use cases • Subclassing a table • Table that has columns of specified UCD • eg Conesearch, SIAP • Aladin • [phot.mag*] < 16 {draw circle [phot.mag]} • exact syntax • experiments with other “template-action” language • generation through GUI • Aladin as a framework – wider development • Other • matching function • template/action

Matching Function • Given two UCD, the matching function expresses “how alike”: • double mu (u1, u2) → [0,1] • I want a table column “like this” • maximize mu over available UCD • How to construct mu? Example: mu ( pos.eq.ra;src.galaxy stat.error;pos.eq.ra )

Concept Property Velocity Class Idea of Galaxy Instance Particular Galaxy: M31 120 km/sec Concept & Property

What is a table class galaxy velocity M31 NGC 3039 M51 120 km/s 350 km/s 448 km/sec instances a table is a set of statements the velocity of M31 is 120 km/s the velocity of NGC3039 is 350 km/s the velocity of M51 is 448 km/s

UCD Tools (from CDS Strasbourg) • Tree browser http://cdsweb.u-strasbg.fr/UCD/tree/js/ • Natural language to UCD http://cdsweb.u-strasbg.fr/UCD/cgi-bin/descr2ucd • Explain a UCDhttp://cdsweb.u-strasbg.fr/UCD/tools.htx

VOTable/VOEvent • Uses UCD to indicate what has been observed <Group name="SQUARE_GALAXY_FLUX"> <Param name="counts" value="73288" unit="ct" ucd="phot.count"/> <Param name="peak" value="1310" unit="ct/s" ucd="arith.rate;phot.count"/> </Group> <Param name="seeing" value="2" unit="arcsec" ucd="instr.obsty.site.seeing"/>

VOEvent • Types of objects • BH, burster, AGN, etc etc • Types of events • SNe, GRB, lensing event, flare, etc etc • Use cases • Shall I observe this/ • Probabilistic interpretation • Has interpretation changed with follow-up?

Semantics • RDF (Resource Description Framework) • statements are triples: • “Resource Related-to Resource” • (Cat Is-instance-of Animal) • (Sigma Is-standard-deviation X) • (RA is-member-of position), (Dec is-member-of position) • T has-UCD phys.temperature • Could use UCD for deduction • Create SED from magnitudes • Grouping parameters to make complex object • eg error ellipse as errX, errY, covXY

subject object predicate Resource or literal Resource Resource RDF Resource Description Framework is for describing and exchanging metadata Example of an RDF statement Roy Williams = Author ( http://www.cacr.caltech.edu/~roy) <rdf:statement> <rdf:subject resource=“http://www.cacr.caltech.edu/~roy”/> <rdf:predicate resource=“http://www.purl.org/dublin-core#author”/> <rdf:object>Roy Williams</rdf:object> </rdf:statement>

Unified Content Descriptors Semantic types for astronomy