Object modelling of temporal changes in Geographical Information Systems
Changes in current temporally enabled GIS systems, thàt have been successfully implemented, are based on the snapshot approach which consist of sequences of discrete images. This approach does not allow either the pattern of changes to be shown or the complexities within the changes to be examined. Also the existing GIS database models cannot represent effectively the history of geographical phenomena. The aim of this research is to develop an object-oriented GIS model (OOGIS) that will represent detailed changes of geographical objects and track the evolution of objects. The detailed changes include spatial, thematic, temporal, events and processes that are involved in the changes. Those have been addressed, but not implemented, by a number of previous GIS projects. Object tracking and evolution includes not only attributes changes to homogenous objects, but also major changes that lead to transforming/destroying existing objects and creating new ones. This will allow the pattern of changes of - geographical phenomena to be examined by tracking the evolution of geographical objects. The OOGIS model was designed using an object-oriented visual modelling tool and was implemented using an object-oriented programming environment (OOPE), an object-oriented database system (OODBS). The visual modelling tool for designing the OOGIS model was Unified Modelling Language (UML), OOPE for implementing the OOGIS model was Microsoft Visual C++ and the OODBS was Objectivity/DB. The prototype of the investigation has been successfully implemented using a Case Study of Royal Borough of Kingston-Upon-Thames, in the United Kingdom. This research is addressing in particular the deficiencies in two existing GIS models that are related to this work. The fust model, the triad model, represents the spatial, thematic and temporal but fails to represent events and processes connected to the changes. The second model, the event-oriented model, though it represents the events (or processes) related to the changes, it stores the changes as attributes of the object. This model is.limited to temporal stable (static) changes and can not be applied to the evolution of geographical phenomena or changes that involve several objects sharing common . .propertíes and temporal relationships. Moreover, the model does not take into account the evolution (e.g. splitting, transformation etc) of a specific object which can involve more than changes to its attributes. Both models are not able to tackle, for instance, in situation when an object such as a park is disappearing to make way for new objects (i.e. roads and new buildings) or in situation where an agriculture piece of land becomes an industrial lot or village becomes a city. In this work the construction of a new approach which overcomes these deficiencies is presented. Also the approach take into account associations and relationships between objects such as inheritance which would be reflected in the object oriented database. For example a road can be regarded a base class from which other classes can be derived such as motorways, streets, dual roads etc which might reflect the evolution of objects ,in non-homogenous ways. The object versioning technique in this work will allow the versions of a geographical object to be related, thereby creating temporal relationships between them. It requires less data storage, since only the changes are recorded. The association between the versions allows continuous forward and backward movement within the versions, and promotes optimum query mechanisms.