Describing OGC WMS and WFS with the OWL-S Web

Service Ontology

Dr Kristin Stock

Allworlds Geothinking, UKCentre for Geospatial Science, University of Nottingham, UK

EDINA, UK

Introduction

• COMPASS Project (http://compass.edina.ac.uk/)

• OGC Standards focus on syntactic descriptions of web services.

• Some recent work incorporates object semantics.

• Nothing so far done on semantics of functionality.

• We tried to fill this gap with OWL-S.

Why do this?

• Web service ontologies are intended to assist with:– Discovery– Dynamic execution– Chaining

• Automating the publish-find-bind model.

OWL-S• One of the dominant web service

ontologies (cf WSMO).• The basic model:

– Profile – advertises what the service does;

– Process Model – describes the process that it executes;

– Grounding – describes how it can be executed.

• Inputs, Outputs, Preconditions, Results

ServiceServiceGrounding

ServiceProfile

ServiceModel

presents(what it does)

supports(how to access it)

describedBy(how it works)

Domain Ontology (Feature Types)

Inputs, Outputs,Preconditions and Results

Inputs, Outputs,Preconditions and Results

GetCapabilities

• Content of GetCapabilities mapped to OWL-S.

• This process could be automated for WFS and WMS.

• Augment with semantic links.

The OWL-S OGC Ontology (1)

• Created an OWL-S OGC Ontology to describe the specifications.

• Each implementation of a WFS or WMS imports the OWL-S OGC Ontology and creates instances.

The OWL-S OGC Ontology (2)

• For WFS and WMS, the OWL-S OGC Ontology is quite comprehensive, because specs are specific – wouldn’t be so for WPS.

OWL-S OGC Ontology: What’s in it?

• Classes for:– WFS and WMS specialised service;– The processes that a WMS or WFS may

offer and inputs and outputs;– An OGCHttp Grounding and operations;– Connections between processes and

the operations that implement them.

The Grounding• OWL-S has a WSDL

grounding.• We didn’t use it because

it’s XML, not OWL – doesn’t fit our architecture.

• Could have applied the WSDL – RDF mapping but time limited.

• Created a new grounding.

OGCHttpGrounding

OGCHttpOperation

OGCHttpParameter

Domain QueryModel

OGCHttpConstraint

hasOGCHttpOperation

hasOGCHttpParameter

hasOGCHttpConstraint

hasDomainhasQueryModelhasDomain

CheckBox

InputBox OptionList

RadioButton

isA

Process:Parameter

groundsAbstractParameter

Children of Process:AtomicProces

s(for each WFS and WMS operation)

groundsAtomicProcess

has<OperationName>Inputhas<OperationName>Output

Meaning

PossibleValues DefaultValu

e

DataType

DomainMetadata

ValuesUnit

hasComponent

How exactly does this describe the semantics?

• IOPR describe semantics of functionality;

• Process describes steps/branches/loops etc. (not implemented)

• IOPR can be connected to concepts in a domain ontology;

• OWL-S OGC ontology includes hasTopic to link service, feature type or layer.

Dynamic service execution

• Grounding includes binding between operation parameters and ‘query model’.

• This is information that can be used to dynamically generate form to ask user for input.

• e.g. Which feature type they would like to query in WFS, layer in WMS.

What’s in the ontology for each web service?• Details from GetCapabilities,

including:– Which operations are implemented;– Actual grounding (URLs) and query

model;– etc.

• Semantic hasTopic links;• You don’t have to repeat IOPR.

What did we find?

• It’s very cumbersome.• Full implementation of OWL-S

includes IOPR more than once (different purposes);

• Difficulties with cardinality constraints.

How did we modify OWL-S?

• Did not describe IOPR multiple times, but once with references, same for parameters.

• New grounding.• Did not define IOPR in each

instance ontology, just referenced.

It’s a limited implementation

• Didn’t fully describe preconditions and results.

• Didn’t fully model the complexity of some parameters.

• Process model limited.

Conclusions

• Full OWL-S implementation impractical.

• OGC specifications make it easier to use OWL-S.

• OWL-S can be used to support discovery and execution.

• Didn’t test orchestration.

Questions or Comments?

or contact me:kristin.stock@nottingham.ac.

uk