Glossary for Geospatial Science

topology or vector topology:� A description of the relationship between node, line, and polygon elements in a vector object.� Three levels of vector topology are available for RVC vector objects: polygonal, planar, and network.� Polygonal topology is the highest, or strictest, level of topology.� Polygonal topology requires that no two nodes have the same X and Y coordinates, all lines start and end in nodes, lines do not intersect without separation by nodes, enclosed areas are defined as polygons, and any point can be in at most one polygon.� Planar topology requires that all lines start and end in nodes and lines do not intersect without separation by nodes, just as for polygonal objects.� However, polygon information is not maintained (which means planar objects have no polygons).� Network topology places nodes at the start and end of all lines, but lines may cross themselves or other lines and there are no polygons.� All three topology types may be either 2D or 3D.� Topology for polygonal and planar 3D objects is maintained in the X-Y plane, which means that polygon dimensions and the location of nodes separating lines that would otherwise cross are determined by projecting onto the X-Y plane.� The constraint imposed by 2D topology on 3D objects is eliminated by network topology, which allows two nodes to have the same X and Y coordinates.

Polygonal topology is necessary for objects such as soil maps and property ownership maps.� Planar topology may be suitable for hydrology in the absence of lakes or road systems that lack underpasses and overpasses or other features that require network topology for correct representation.