Polygon Auto-Construction Algorithm Based on Vector External Product of Virtual Arc

Various vector data to establish topological relation and association are the key issues of geographic information system, patch of the surface, that is polygon, whose generation is the core of establishing topology. This paper put forward auto-construction algorithm is to judge polygon´s direction that based on azimuth left turn algorithm. Patch (polygon) auto-construction algorithm calculates azimuth, turning left to construct polygon. In the judge of polygon direction, establishing virtual arcs, calculating all of their relative vector external product, judging polygonal search direction is left or right, to certify search direction is unique.