1. A method of viewing map data in a computer-implemented geographic information system, comprising the steps of: retrieving a map window file from a data storage device coupled to a computer, the map window file containing map definition information that identifies vector-based map data used to generate a map picture, the vector-based map data being comprised of primitives that define map objects, each map object defining one or more components of the map picture; generating an initial map picture with vector-based map data identified by the map definition information; displaying the generated initial map picture on a display device coupled to a computer; receiving a request for additional information for the displayed initial map picture; determining a location of the vector-based map data required to satisfy the received request by obtaining map definition information from the map window file; retrieving the additional vector-based map data from a data storage device on a computer; generating a new map picture based on the retrieved vector-based map data; and displaying the generated new map picture on the display device coupled to a computer.