Graph is a subclass of GeographicObject. An object belonging to Graph contains a set of connections between geographic objects. These connections are defined as relationships. For example, a graph may establish the connections between destinations for a given airline. Other common examples are road and stream networks; a special type of network is a single line network, appropriate for modelling these examples. Users are expected to define subclasses of Graph where appropriate for their applications. Some possible subclasses of Graph include: AirlineRoutes, StreamNetwork and HighwaySystem. Note that geographic objects referenced in a graph (like the airline destinations example) need not be physically connected.