Data Models for Object-Component Technology

Geographic Data Modeling Background
A data model is a set of constructs for describing and representing parts of the real world in a computer system. Data models are vitally important to GIS because they control the way that data are stored and have a major impact on the type of analytical operations that can be performed. Early GIS were based on CAD, simple graphical and image data models. In the 1980s and 1990s the hybrid geo-relational model came to dominate GIS. In the last few years major software systems have been developed based on more advanced and standardized geographic object data models that include elements of all earlier models. Data Model; Application

CAD	Engineering design
Graphical (non-topological)	Simple mapping
Image	Image processing
Raster/grid	Spatial analysis and modeling
TIN	Surface/terrain analysis and modeling
Geo-relational	Geo-processing on geometric features
Object	Geo-processing on custom features with behavior
Object-component	Extensible geo-processing on custom features with behavior

Figure 1: AM/FM/GIS data models and their applications.

What is a Data Model?
The heart of any GIS is the system-level data model, which is a set of constructs for representing objects and processes in the digital environment of the computer (Figure 2). People interact with operational GIS in order to perform tasks like making maps, querying databases and performing site suitability analyses. Because the types of analyses that can be undertaken are strongly influenced by the way the real world is modeled, decisions about the type of model to be adopted are vital to the success of a GIS project. Geographic reality is infinitely complex, but computers are finite. Therefore, difficult choices have to be made about what and how things are modeled. Because different types of people use GIS for different purposes, and the type of phenomena people study have different characteristics, there is no single type of data model that is best for all circumstances.

Figure 2: The role of a data model in GIS.

Levels of Data Model Abstraction
When representing the real world in a computer, it is helpful to think in terms of the four different levels of abstraction (levels of generalization or simplification) that are shown in Figure 3. First, reality is made up of real world phenomena (buildings, streets, wells, lakes, people, etc.), and includes all aspects that may or may not be perceived by individuals, or deemed relevant to a particular application. Second, the conceptual model is a human-oriented, often partially structured, model of selected objects and processes that are thought relevant to a particular problem domain. Third, the logical model is an implementation-oriented representation of reality that is often expressed in the form of diagrams and lists. Fourth, the physical model is a slightly misleading term for the digital abstraction that portrays the actual application in a computer system, and often comprises tables stored as files or databases. Use of the term physical here is actually misleading because the models are not actually physical, but only exist digitally in computers.

In data modeling, users and system developers participate in a process that successively engages with each of these levels. The first phase of modeling begins with a definition of the main types of objects to be represented in the GIS and concludes with a conceptual description of the main types of objects and relationships between them. Once this phase is complete, further work will lead to the creation of diagrams and lists describing the names of objects, their behavior, and the type of interaction between objects. This type of logical data model is very valuable for defining what a GIS will do and the type of domain over which it will extend. Logical models are implementation independent, and can be created in any GIS with appropriate capabilities. The final data modeling phase involves creating a model showing how the objects under study can be digitally implemented in a GIS. Physical models describe the exact files or database tables used to store data, the relationships between object types and the precise operations that can be performed.

Figure 3: Levels of abstraction relevant to GIS data models.

A data model provides system developers and users with a common understanding and reference point. For developers a data model is the means to represent an application domain in terms that may be translated into a design and implementation of a system. For users, it provides a description of the structure of the system, independent of specific items of data or details of the particular application.