An Outline of an Event-driven Spatial Data Structure for Managing Time-varying Maps

Traditional cartography, when managing change in the real world being mapped naturally turned to the time-slice concept: “This is what the real world looked like at date X”. Computer based replacements for manual techniques used the same approach. The reasons for preserving the time-slice mode! were partly for compatibility with previous operations, and partly because the computer methodologies were inherently batch-oriented. In other words, any modification to a polygon-based map, for example. required the complete reconstruction of the topology the linkages between polygons, arcs and nodes: the technology had to recommence with detecting all line intersections. and then rebuilding a!! the polygons. More recently, this time-slice approach has been questioned, especially when a complete history of all changes is required, for example in historical atlases or in the mapping of forest harvesting operations, or whenever a request is made to retrieve the information about the real-world situation at an time of interest. This requires that the spatial data base preserve date-stamped events, and the spatial relations among map objects at any past time. This paper addresses the issue of maintaining a history of topological events those spatial occurrences that change the spatial adjacency relationships among t h e map objects. The concept of a topological event is based on the incremental (as opposed to the batch) mode! of topology construction, where the temporal sequence of the actions of the cartographer’s pen is reproduced in the building of the topological mesh. This is not considered feasible with the usual line intersection model of space, but has been developed with the Voronoi spatial model. The maintenance of a history of topological events then becomes a question of recording each topological update and its time of occurrence. together with the linkages back to the unchanged portion of the map. The management of the history of the modification of the map o b j e c t s (e.g. line segments) may be done with the usual data base methods.