Thiessen Polygons

Applicability:

• Cadcorp SIS Map Modeller
• Cadcorp SIS ActiveX Modeller

Ribbon Group:

Analysis-Buffer

Conditions for use:

• Item Class: All item classes
• Minimum number selected: 2
• Minimum item status: Editable

Associated commands:

• Buffer

Also see:

• Buffer Fence
• Circle Fence
• Offset Fence
• Polygon Fence

Description:

 Thiessen Polygons creates areas of influence around the selected items. Each selected item will generate a Polygon item.

Thiessen Polygons (also known as Voronoi, 1909, or Dirichlet cells, 1850) are Polygons created automatically, based on selected items. Adjacent Polygon items are created whose boundaries are equidistant from the selected items.

Creating Thiessen Polygons (Map Modeller only)

Thiessen Polygons are created as Polygon items on the current overlay.

1. Make sure the overlay on which you want to create the Thiessen Polygons is the current overlay (you could consider creating a separate overlay for these items).
   
   
2. Select the items around which to create areas of influence. There must be at least two of them.
   
   
3. Select Thiessen Polygons.

Example:

As an example of the use of Thiessen Polygons consider the following:.

A city is holding elections. There will be many polling stations where people can walk to vote. Each house must be allocated to a polling station, but you want to make sure that each house is allocated to the closest one. To do this follow this procedure:

1. Create a Point item at each polling station.
   
   
2. Select the polling stations.
   
   
3. Use Thiessen Polygons to create the Thiessen Polygons.
   
   This will give you the Polygon (area) which each polling station will collect voters from.

4. For each Polygon, use Find [Home-Selection] to find all the houses inside that area.

This geometrical construction is sometimes called a Voronoi, or Thiessen construction. It is calculated by creating the dual network of the Delaunay triangulation.

Editing Thiessen Polygons

Once constructed, a Thiessen Polygon is a Polygon item, and can be edited in the same way as any Polygon item - you can change its size and shape, its pen and brush, and so on. See Constructing Polygon items and Editing Polygon items.

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