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web. The VPFTableHeader class provides the schema for each table that is 
(A) Metadata Classes (C) Spatial Index (Quadtree) C fasses 
Figure 1. Definitions and Relationships of OVPF Classes 
necessary to fully define each feature and its relationship to other features within the 
coverage, library and database. All the arrows used in the figure represent pointers. 
Instead of the expensive table joins and more difficult referential integrity concerns, 
pointers are used in object-oriented technology to encapsulate all information of a feature. 
Instance variables are data structures for which each instance-object has its own private 
copy. Class instance variables are data structures for which the class object and each of its 
subclasses are defined to have a private copy of the variable. These variables can hold a 
subclass-specific default value for a constant that can be accessed with the same name from 
any class in the hierarchy. This helps to reduce “variable-name vocabulary.” Class 
variables are data structures for which the defining class has a single copy, that can be 
directly accessed by all its instances. For more detailed information, see (ParcPlace 1994). 
A sample of OVPF objects for a Digital Nautical Chart (DNC) Coastline feature is 
shown in Figure 2. For more detailed study in the topic of object-oriented technology, see 
(Arctur 1995, Goldberg 1989, Rumbaugh 1991, Booch 1994).