MySQL 5.6 Reference Manual Including MySQL NDB Cluster 7.3-7.4 Reference Guide
Geometry
is the root class of the
hierarchy. It is a noninstantiable class but has a number of
properties, described in the following list, that are common
to all geometry values created from any of the
Geometry
subclasses. Particular subclasses
have their own specific properties, described later.
Geometry Properties
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 an integer associated with the geometry value. The maximum usable SRID value is 232−1. If a larger value is given, only the lower 32 bits are used. All computations are done assuming SRID 0, regardless of the actual SRID value. SRID 0 represents an infinite flat Cartesian plane with no units assigned to its axes.
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 geodetic 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 (LineString
,
MultiPoint
,
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 (LineString
,
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 MultiPoint
,
MultiLineString
, and
MultiPolygon
objects are the same as
the dimensions of the elements they consist of.