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.ra alpha;

UCD is a semantic type (not a name)

pos.eq.ra pos.eq.dec pos.eq.ra pos.eq.dec

alpha1 delta2delta1 alpha2name

ucd

example: tablae of double stars

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 UCD

DataModel

Richer, Fuzzier Thinner, Exact

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-partstruct 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

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.

Multiple words

Quantity UCD

Magnitude phot.mag

Flag (weather, reliability, etc)

meta.code; phot.mag

Error on magnitude stat.error; phot.mag

Maximum temperature of an instrument

phys.temperature; instr; stat.max

Error of a V-band magnitude stat.error; phot.mag; em.opt.V

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:

meta.id pos.eq.ra pos.eq.dec

blahblah bananafoo bar syzygy

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.galaxystat.error;pos.eq.ra

)

Concept & Property

Idea of Galaxy

Particular Galaxy: M31

Velocity

120 km/sec

Class

Instance

Concept Property

What is a table

galaxy velocity

M31NGC 3039M51

120 km/s350 km/s448 km/sec

the velocity of M31 is 120 km/sthe velocity of NGC3039 is 350 km/sthe velocity of M51 is 448 km/s

class

instancesa table is aset of statements

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

RDF Resource Description Frameworkis for describing and exchanging metadata

Roy Williams = Author ( http://www.cacr.caltech.edu/~roy)

Example of an RDF statement

predicate subjectobject

<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>

Resourceor literal

Resource Resource