The geometry classes define a hierarchy as follows:
It is not possible to create objects in non-instantiable classes. It is possible to create objects in instantiable classes. All classes have properties, and instantiable classes may also have assertions (rules that define valid class instances).
Geometry is the base class. It is an abstract
class. The instantiable subclasses of
Geometry are restricted to zero-, one-, and
two-dimensional geometric objects that exist in two-dimensional
coordinate space. All instantiable geometry classes are defined
so that valid instances of a geometry class are topologically
closed (that is, all defined geometries include their boundary).
Geometry class has subclasses for
Point represents zero-dimensional
Curve represents one-dimensional objects,
and has subclass
Surface is designed for two-dimensional
objects and has subclass
GeometryCollection has specialized zero-,
one-, and two-dimensional collection classes named
MultiPolygon for modeling geometries
corresponding to collections of
MultiSurface are introduced as abstract
superclasses that generalize the collection interfaces to
MultiSurface are defined as non-instantiable
classes. They define a common set of methods for their
subclasses and are included for extensibility.
MultiPolygon are instantiable classes.