Chapter 3 Object Oriented Systems and Open GIS

Objectives of the Chapter Establish place of O-O in OpenGIS cover basics of O-O emphasise design issues of O-O provide overview of complex technical issues of O-O inOpenGIS illustrate O-O system

Objectives of the Chapter show why O-O is important in OpenGIS provide example of O-O system set exercise in O-O design for GIS link to other OpenGIS issues

Structure of the Chapter introduction basics of O-O use of O-O merits of O-O project methods a sample system

Structure of the Chapter Technical sytems background on –interoperability and O-O –geosystems and O-O OpenGIS –systems design in O-O environment –system examples of O-O use –related issues in an open environment

O-O in general The nature of object oriented thinking the use of objects throughout the analysis and design stages the benefits of objects

What are O-O Systems Source of materials overview of approach to O-O analysis design

What are O-O Systems Characterisitcs of an O-O system are: abstraction encapsulation hierarchies communication via messages

What are O-O Systems? Object Orientation is not defined by O-O language but by the four characteristics O-O design is the key Design preceeds coding

Object Statics Instances Classes Attributes defaults probability multiplicity qualifiers constraints derived attributes

Object Relations Relationships arity Features cardinality qualifiers Constraints Parametric realtional Instances Collections

Object Dynamics Problem of Describing Behaviour Transition networks Parallelism Ensembles States

Object Interaction One and two way interaction Guards Transitions and events Sending and Receiving Protocols

O-O Software Engineering Basic elemenets of methods Waterfall Model O-O Methods spcifics O-O Methods types O-O Methods sequences of stages

O-O Analysis User Perspectives in Analysis Problem Specification Object Discovery –evaluation Object hierarchies and attributes Object messages and methods

O-O from Analysis to Design Ideally with an O-O approach analysis and design mesh seamlessly. The Design follows a series of: –transformations which correspond to –design phases

O-O from Analysis to Design Design Criteria based on classes basic issue is coupling basis of interoperability set of design issues related to »representation »values »protocols »code ruling ideas are cohesion and reliability

O-O from Analysis to Design Management of Design –Roles support teams –Reuse and Tools controling components evaluating components evaluating use

Method Default steps –requirements –system context –sub-systems –vocabulary –classes and relations –models Alternative Approaches

Project Methods Approaches to system analysis and design are based in genral on the waterfall model this is weak in many respects alternative approches are based on O-O and other technologies which allow rapid modification and re-use and therefore demand a different sequence of design stages and allow prototyping

Relative Merits of O-O Considered in relation to system functions and to business benefits system fucntion benefits realte to compatability and ease of use business benefits are manifold realting to better design and more focussed operations which produce effectiveness and efficiency gains

Object-Orientation and OpenGIS What does O-O mean for GIS?

OGIS and O-O Basics of Interoperable GIS –Interoperability by means of specification manufacturers comliance with Open criteria these are based on the object model at each design level –Open systems are based on Distributed Computing Platforms OGIS will be independent of these this is the challenge!!

OGIS and O-O –based on O-O ideas and technology –but requiring a hybrid approach key issues are – encapsulation –inheritance

OGIS and O-O OGIS model is – Pluggable Computing Model –high level design model –basis for links to non-spatial software, services and systems

Figure 4-4

Figure 4-5

Services in the Pluggable Component Model Human-Technology Interface Services Tool Services Data Management Services Distributed Computing Platform (DCP) Services Operating System Services Hardware Platforms

OGIS and O-O Benefits of the Pluggable Computing Model –clarity of design issues –resource sharing –data transfer –enhanced functionality –incorporate legacy systems

Benefits of model Scalability extendibility diversity interoperability

OGIS and O-O The Open Geodata Model –levels of abstraction –entities and phenomena

Geodata model- time/ location

Features and Coverages Views of the world

Coverages and features

Components of features Geometry properties metadata

Definitions of elements Associated with elements are detailed definitions open geoprocessing and interoperability rely on definitions and their use the open model can cope with some inconsistencies

OGIS community model Making it work requires agreement this is a question of organisation and of designing agreeable specifications it is all a question of the information community and how it works

Community and semantics Five assumptions of community semantics

Community and semantics The other three assumptions

What does this mean for Open GIS and O-O Semantics is at the heart of the open GIS problem issues of design are bound up with user definitions business benefits are bound up with openness and interoperability therefore semantic issues and translators are vital

Design issues - an example See the paper by Peter Batty on O-O Some objectivity please. gives some details of design questions and the role of O-O Also see the paper on legacy data and its challenges

Design issues - second example See the paper by Gebhart and Porter which raises the key issues for what is perhaps the most difficult area for interoperability and open systems - legacy data

Open systems To date there have been no true interoperable systems the creation of the vision is a long term objective elements of the vision exist in network systems and partial open functionality

example One example of a network system is Map Objects Internet Map Server Map Objects has a object structure seen in its system diagram this structure is basically the same as classic Arc Info but translated into object form it operates over the internet visit the diagram and the web site to familiarise yourself with them

Example An example of partial interoperable GIS is Geomedia already the leader but not fully interoperable visit the web site and use it in the practical classes

Exercise