Overview of Polygons
Concept Procedure Quick Reference

A polygon is an object type with closed boundaries. Polygons store information about their inner and outer boundaries, and about other polygons nested within them or grouped with them.

Polygons can represent areas such as city limits, county boundaries, state borders, buildings, and parcels, as well as more complex objects, such as islands.

Example: A state map could be composed of a single polygon with an outer boundary representing the state, interior boundaries representing lakes, and boundaries within those boundaries representing islands. A country map could be composed of individual polygons representing each state.

The following table defines common terms used to describe the structure of polygons.

Term Definition
Boundaries Closed boundaries that make up a polygon. Polygons can have multiple non-intersecting boundaries, or boundaries nested within boundaries.
Balancing Process of recalculating which boundaries are outer or inner. Nested boundaries are alternately classified as outer and inner. That is, the outermost boundary is classified as an outer boundary. A boundary nested within this boundary is an inner boundary. A boundary nested within the inner boundary is classified as an outer boundary.
Inner boundary Nested boundary that is totally within an outer boundary.
Outer boundary The outermost boundary for any discreet set of boundaries that define the polygon, or a boundary residing within an inner boundary. A polygon can have several unnested outer boundaries and several nested outer boundaries.

Understanding Boundaries

The figure below shows two polygon objects, each with three boundaries. The one on the left has two discrete outer boundaries and one inner boundary. The inner boundary is nested within the second discrete outer boundary. The polygon on the right also has two outer boundaries and one inner boundary. However, the second outer boundary is nested within the inner boundary.

Polygon objects maintain a tree structure to keep track of the boundaries and identify nesting levels. The illustration below shows the different tree structures for the two objects shown above. The first polygon tree contains two branches, while the second polygon tree contains a single branch.

In addition to outer and inner boundaries, there is an Annotation boundary type. This boundary has the characteristics of an inner boundary, but only affects the display of the pattern fill and is ignored when calculating the area or interior of the polygon object. Its primary purpose is to allow you to annotate your drawings without the fill pattern of the polygon obscuring the annotations. The annotation will typically consist of text or blocks.

See Also