About Polygon Objects
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.
Polygons can contain other polygons within them.
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.
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