14.16.7.4 Polygon and MultiPolygon Property Functions

Functions in this section return properties of Polygon or MultiPolygon values.

Unless otherwise specified, functions in this section handle their geometry arguments as follows:

• If any argument is NULL or any geometry argument is an empty geometry, the return value is NULL.

• If any geometry argument is not a syntactically well-formed geometry, an ER_GIS_INVALID_DATA error occurs.

• If any geometry argument is a syntactically well-formed geometry in an undefined spatial reference system (SRS), an ER_SRS_NOT_FOUND error occurs.

• For functions that take multiple geometry arguments, if those arguments are not in the same SRS, an ER_GIS_DIFFERENT_SRIDS error occurs.

• Otherwise, the return value is non-NULL.

These functions are available for obtaining polygon properties:

• ST_Area({poly|mpoly})

  Returns a double-precision number indicating the area of the Polygon or MultiPolygon argument, as measured in its spatial reference system.

  ST_Area() handles its arguments as described in the introduction to this section, with these exceptions:

  • If the geometry is geometrically invalid, either the result is an undefined area (that is, it can be any number), or an error occurs.

  • If the geometry is valid but is not a Polygon or MultiPolygon object, an ER_UNEXPECTED_GEOMETRY_TYPE error occurs.

  • If the geometry is a valid Polygon in a Cartesian SRS, the result is the Cartesian area of the polygon.

  • If the geometry is a valid MultiPolygon in a Cartesian SRS, the result is the sum of the Cartesian area of the polygons.

  • If the geometry is a valid Polygon in a geographic SRS, the result is the geodetic area of the polygon in that SRS, in square meters.

  • If the geometry is a valid MultiPolygon in a geographic SRS, the result is the sum of geodetic area of the polygons in that SRS, in square meters.

  • If an area computation results in +inf, an ER_DATA_OUT_OF_RANGE error occurs.

  • If the geometry has a geographic SRS with a longitude or latitude that is out of range, an error occurs:

    • If a longitude value is not in the range (−180, 180], an ER_GEOMETRY_PARAM_LONGITUDE_OUT_OF_RANGE error occurs.

    • If a latitude value is not in the range [−90, 90], an ER_GEOMETRY_PARAM_LATITUDE_OUT_OF_RANGE error occurs.

    Ranges shown are in degrees. The exact range limits deviate slightly due to floating-point arithmetic.

  mysql> SET @poly =
         'Polygon((0 0,0 3,3 0,0 0),(1 1,1 2,2 1,1 1))';
  mysql> SELECT ST_Area(ST_GeomFromText(@poly));
  +---------------------------------+
  | ST_Area(ST_GeomFromText(@poly)) |
  +---------------------------------+
  |                               4 |
  +---------------------------------+
  
  mysql> SET @mpoly =
         'MultiPolygon(((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1)))';
  mysql> SELECT ST_Area(ST_GeomFromText(@mpoly));
  +----------------------------------+
  | ST_Area(ST_GeomFromText(@mpoly)) |
  +----------------------------------+
  |                                8 |
  +----------------------------------+

• ST_Centroid({poly|mpoly})

  Returns the mathematical centroid for the Polygon or MultiPolygon argument as a Point. The result is not guaranteed to be on the MultiPolygon.

  This function processes geometry collections by computing the centroid point for components of highest dimension in the collection. Such components are extracted and made into a single MultiPolygon, MultiLineString, or MultiPoint for centroid computation.

  ST_Centroid() handles its arguments as described in the introduction to this section, with these exceptions:

  • The return value is NULL for the additional condition that the argument is an empty geometry collection.

  • If the geometry has an SRID value for a geographic spatial reference system (SRS), an ER_NOT_IMPLEMENTED_FOR_GEOGRAPHIC_SRS error occurs.

  mysql> SET @poly =
         ST_GeomFromText('POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7,5 5))');
  mysql> SELECT ST_GeometryType(@poly),ST_AsText(ST_Centroid(@poly));
  +------------------------+--------------------------------------------+
  | ST_GeometryType(@poly) | ST_AsText(ST_Centroid(@poly))              |
  +------------------------+--------------------------------------------+
  | POLYGON                | POINT(4.958333333333333 4.958333333333333) |
  +------------------------+--------------------------------------------+

• ST_ExteriorRing(poly)

  Returns the exterior ring of the Polygon value poly as a LineString.

  ST_ExteriorRing() handles its arguments as described in the introduction to this section.

  mysql> SET @poly =
         'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
  mysql> SELECT ST_AsText(ST_ExteriorRing(ST_GeomFromText(@poly)));
  +----------------------------------------------------+
  | ST_AsText(ST_ExteriorRing(ST_GeomFromText(@poly))) |
  +----------------------------------------------------+
  | LINESTRING(0 0,0 3,3 3,3 0,0 0)                    |
  +----------------------------------------------------+

• ST_InteriorRingN(poly, N)

  Returns the N-th interior ring for the Polygon value poly as a LineString. Rings are numbered beginning with 1.

  ST_InteriorRingN() handles its arguments as described in the introduction to this section.

  mysql> SET @poly =
         'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
  mysql> SELECT ST_AsText(ST_InteriorRingN(ST_GeomFromText(@poly),1));
  +-------------------------------------------------------+
  | ST_AsText(ST_InteriorRingN(ST_GeomFromText(@poly),1)) |
  +-------------------------------------------------------+
  | LINESTRING(1 1,1 2,2 2,2 1,1 1)                       |
  +-------------------------------------------------------+

• ST_NumInteriorRing(poly), ST_NumInteriorRings(poly)

  Returns the number of interior rings in the Polygon value poly.

  ST_NumInteriorRing() and ST_NuminteriorRings() handle their arguments as described in the introduction to this section.

  mysql> SET @poly =
         'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
  mysql> SELECT ST_NumInteriorRings(ST_GeomFromText(@poly));
  +---------------------------------------------+
  | ST_NumInteriorRings(ST_GeomFromText(@poly)) |
  +---------------------------------------------+
  |                                           1 |
  +---------------------------------------------+