The OpenGIS Geometry Model •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 2 32 − 1. If a larger value is given, only the lower 32 bits are used. SRID 0 represents an infinite flat Cartesian plane with no units assigned to its axes. To ensure SRID 0 behavior, create geometry values using SRID 0. SRID 0 is the default for new geometry values if no SRID is specified. For computations on multiple geometry values, all values must have the same SRID or an error occurs. 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. 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. ((MINX MINY, MAXX MINY, MAXX MAXY, MINX MAXY, MINX MINY)) 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. •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. •Its  interior ,  boundary , and  exterior . •Its  MBR  (minimum bounding rectangle), or envelope. This is the bounding geometry, formed by the minimum and maximum (X,Y) coordinates: •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. 2053