Geometry is the root class of the hierarchy.
It is a noninstantiable class but has a number of properties
that are common to all geometry values created from any of the
Geometry subclasses. These properties are
described in the following list. Particular subclasses have
their own specific properties, described later.
A geometry value has the following properties:
Its type. Each geometry belongs to one of the instantiable classes in the hierarchy.
Its SRID, or Spatial Reference Identifier. This value identifies the geometry's associated Spatial Reference System that describes the coordinate space in which the geometry object is defined.
In MySQL, the SRID value is just an integer associated with the geometry value. All calculations are done assuming Euclidean (planar) geometry.
Its coordinates in its Spatial Reference System, represented as double-precision (8-byte) numbers. All nonempty geometries include at least one pair of (X,Y) coordinates. Empty geometries contain no coordinates.
Coordinates are related to the SRID. For example, in different coordinate systems, the distance between two objects may differ even when objects have the same coordinates, because the distance on the planar coordinate system and the distance on the geocentric system (coordinates on the Earth's surface) are different things.
Its interior, boundary, and exterior.
Every geometry occupies some position in space. The exterior of a geometry is all space not occupied by the geometry. The interior is the space occupied by the geometry. The boundary is the interface between the geometry's interior and exterior.
Its MBR (Minimum Bounding Rectangle), or Envelope. This is the bounding geometry, formed by the minimum and maximum (X,Y) coordinates:
((MINX MINY, MAXX MINY, MAXX MAXY, MINX MAXY, MINX MINY))
Whether the value is simple
or nonsimple. Geometry
values of types (
MultiLineString) are either simple or
nonsimple. Each type determines its own assertions for being
simple or nonsimple.
Whether the value is closed
or not closed. Geometry
values of types (
MultiString) are either closed or not
closed. Each type determines its own assertions for being
closed or not closed.
Whether the value is empty
or nonempty A geometry is
empty if it does not have any points. Exterior, interior,
and boundary of an empty geometry are not defined (that is,
they are represented by a
NULL value). An
empty geometry is defined to be always simple and has an
area of 0.
Its dimension. A geometry can have a dimension of –1, 0, 1, or 2:
–1 for an empty geometry.
0 for a geometry with no length and no area.
1 for a geometry with nonzero length and zero area.
2 for a geometry with nonzero area.
Point objects have a dimension of zero.
LineString objects have a dimension of 1.
Polygon objects have a dimension of 2.
The dimensions of
MultiPolygon objects are the same as the
dimensions of the elements they consist of.
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