Creating and Validating Object-Oriented Geographic Data Models: Modeling Flow within GIS

Object-oriented geographic data models provide an organizational scheme to associate domain specific meaning to primitive GIS elements like points, polylines, and polygons. Although use of data models is widespread in the GIS community, the design process is not necessarily obvious and often ad hoc. This article outlines a procedure for the creation and validation of geographic data models through the examination and distillation of use cases. As an example, the article follows the development of a data model for the spatial concept of flow. Flow, the collective movements of people, materials, or ideas, is a common driver of geographic change, not generally supported by functionality within contemporary GIS, and an abstract dynamic entity that would presumably be difficult to model. Model design was facilitated through the distillation of flow cases of tabular human migration data, Minard's map of Napoleon's march on Moscow, and stream channel routes in a karst watershed. Unified Modeling Language diagrams are created for each case and the models' commonalities combined to yield a generic data model. As a means of validation, each use case was instantiated with the generic model and tested to re-create the fundamental components of flow and address predefined typical queries.