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Unified Content Descriptors Semantic types for astronomy Roy Williams, California Institute of Technology.

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1 Unified Content Descriptors Semantic types for astronomy Roy Williams, California Institute of Technology

2 History Created at CDS Strasbourg (the first VO prototype) Harvested From 5000 tables, 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.ra alpha;

3 UCD is a semantic type (not a name) pos.eq.rapos.eq.decpos.eq.rapos.eq.dec alpha1delta2delta1alpha2 name ucd example: tablae of double stars

4 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 Natural Language UCD Data Model Richer, Fuzzier Thinner, Exact

5 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

6 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

7 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

8 UCD Tree arith Quantities related to arithmetic and mathematics, including count, difference, ratio em The electromagnetic spectrum meta Quantities related to metadata, such as identifiers, flags, notes, URL, and instr Quantities related to an instrument; typical sub-levels are telescope, observatory, etc. obs Observation methods such as detector, filter, plate, spectrograph, exposure time, etc. phot All photometric measurements, organized according to the wavelength; includes polarization. phys Generic physical quantities, such as length, velocity, mass, and including atomic & molecular concepts and properties, temperature, pressure, gravity, etc... pos Position in the sky, reference frames; including equatorial, galactic etc coordinates; geocentric, heliocentric etc; and precession and nutation. spect Quantities related to spectroscopic measurements src Properties of the observed source of radiation: source classifications and morphology, extension in the sky, variability, stat Statistical quantities and quantities related to model fitting, including concepts such as error, maximum, residuals. time Quantities related to time.

9 Multiple words QuantityUCD Magnitudephot.mag Flag (weather, reliability, etc) meta.code; phot.mag Error on magnitudestat.error; phot.mag Maximum temperature of an instrument phys.temperature; instr; stat.max Error of a V-band magnitudestat.error; phot.mag; em.opt.V First word: what semantic type is this thing? -- other words are helpers

10 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?

11 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)

12 Cone Search service request is a cone service response is a table: meta.idpos.eq.rapos.eq.dec blahblahbananafoobarsyzygy Must have 3 columns with these UCDs May be other columns

13 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

14 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 )

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

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

17 UCD Tools (from CDS Strasbourg) Tree browser Natural language to UCD Explain a UCD

18 VOTable/VOEvent Uses UCD to indicate what has been observed

19 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?

20 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

21 RDF Resource Description Framework is for describing and exchanging metadata Roy Williams = Author ( Example of an RDF statement predicate subject object Roy Williams Resource or literal Resource

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