The interior of a geometry is all points that are part of the geometry except the boundary.
The exterior of a geometry is all points that are not part of the geometry. This can include the space inside an interior ring, for example in the case of a polygon with a hole. Similarly, the space both inside and outside a linestring ring is considered the exterior.
The boundary of a geometry is what is returned by the ST_Boundary method.
Knowing the boundary of a geometry helps when comparing to another geometry to determine how the two geometries are related. However, while all geometries have an interior and an exterior, not all geometries have a boundary, nor are their boundaries always intuitive.
Here are some cases of geometries where the boundary may not be intuitive:
Point A point (such as A) has no boundary.
Lines and curves The boundary for lines and curves (B, C, D, E, F) are their endpoints. Geometries B, C, and E have two end points for a boundary. Geometry D has four end points for a boundary, and geometry F has four.
Polygon The boundary for a polygon (such as G) is its outer ring and any inner rings.
Rings A ring—a curve where the start point is the same as the end point and there are no self-intersections (such as H)—has no boundary.
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