# How Thiessen works

This topic applies to ArcInfo only.

The Thiessen tool can proportionally divide and distribute a point coverage into regions known as Thiessen or Voronoi polygons.

The Thiessen polygons are constructed as follows:

- The input points are scanned left to right and top to bottom. Points closer than the chosen proximal tolerance to previously scanned points are ignored.
- All points are triangulated into a triangulated irregular network (TIN) that meets the Delaunay criterion.
- The perpendicular bisectors for each triangle edge are generated, forming the edges of the Thiessen polygons. The locations at which the bisectors intersect determine the locations of the Thiessen polygon vertices.
- The Thiessen polygons are built to generate polygon topology. The locations of the points are used as the label points for the Thiessen polygons.

Published 6/8/2010