In short, there is a large and diverse base of users for GIS-T. They have different objectives toward which they work, and they have different constraints on the transportation activities they can conduct.
3. GIS_T Data models
In order to describe the structure of a database, it is required to know the concept of data model. A data model is a collection of conceptual tools for describing data, data relationships, data semantics, and the data constraints.
In practice, several sets of GIS-T data models have evolved over time. For the purposes of the very brief review provided here we divide these models loosely into three groups: Network Models, Process Models, and Object Models. [Page]
By far the most prolific network structure for GIS-T applications has been the model accepted by the U.S. Bureau of the Census for its Topologically Integrated Geographic Encoding and Referencing (TIGER) files. The TIGER model had developed from some earlier network data models, and it was marked by its adherence to the principle of planar enforcement. Planar enforcement simply means that all lines in the network are forced into a single plane, and all intersections of lines are defined in that plane. the planar enforced TIGER model presented several difficulties. First, many transportation applications are not concerned with the polygons that may have transportation features as their boundaries. It is the transportation features themselves that are of interest. Secondly, the planar enforcement that was needed to generate polygons also had the effect of splitting transportation features into many small segments whenever two features crossed in the plane. Therefor
