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A 3D Spatial Data Model for Terrain Reasoning

TROTT, Kevin C. (kevin_trott@partech.com) and GREASLEY, IAN (ian_greasley@partech.com), PAR Government Systems Corporation, 314 S. Jay Street, Rome, NY 13440

Key Words: topology, 3-D, volumes, vector data, NIMA, VPF

This paper describes an object-oriented 3-D spatial data model, based on the National Imagery and Mapping Agencyís (NIMA) Vector Product Format (VPF), which is capable of supporting high-resolution 3-D representations of natural and man-made environments with full 3-D topology. The development of this spatial data model was driven by the requirements of creating timely, accurate, and highly detailed synthetic environment databases that provide realistic representations of real-world locations for military training and mission rehearsal purposes. Simulated, computer-generated military forces must be able to perform topological reasoning on the contents of such databases in order move and fight in a realistic manner. These databases often violate the assumptions that are relied upon by traditional 2-D digital topographic databases. For example, the assumption that elevation has a single value at a specified 2-D location does not necessarily hold true. Structures such as bridges, overpasses, tunnels, and the interiors of buildings cannot be adequately represented using 2-D topology; therefore, a spatial data model that supports 3-D topology is needed.

Previous work in the development of data models that support 3-D topology, from both the GIS and computer graphics communities, is briefly reviewed. The 3-D topological data model is then described in detail. The spatial entities that make up the data model, including nodes, edges, faces, and volumes, are each defined. Organizational groupings of spatial entities, including rings, which group and order the edges that bound a face, and shells, which group the faces that bound a volume, also are defined.

All of the topological relationships among the 3-D spatial entities are then defined. The 3-D topological relationship between a node and its collection of connected edges is no longer ordered, since the relationship between edges and faces becomes much more complex than in 2-D planar topology. Each edge has an ordered collection of zero, or more, adjacent faces, while each face is bounded by an ordered collection of one or more edges. Two additional levels of topology, beyond planar topology, are defined: Level 4, which provides partial 3-D topology, in which the rules of planar topology no longer fully hold, and Level 5, which provides full 3-D topology, in which space is partitioned into a collection of mutually exclusive and exhaustive volumes. An extended set of access primitives to support 3-D topological operations also is defined.

Volume features, such as buildings and bodies of water, are defined and discussed. Several examples are presented that illustrate the use of the 3-D topology data model to represent situations that cannot be adequately represented using 2-D planar topology.