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16.5.2.6 MultiPolygon Functions »
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16.5.2.5. Polygon Functions

These functions return properties of Polygon values.

  • Area(poly)

    Returns as a double-precision number the area of the Polygon value poly, as measured in its spatial reference system. 

    mysql> SET @poly = 'Polygon((0 0,0 3,3 0,0 0),(1 1,1 2,2 1,1 1))';
mysql> SELECT Area(GeomFromText(@poly));
+---------------------------+
| Area(GeomFromText(@poly)) |
+---------------------------+
|                         4 |
+---------------------------+

  • ExteriorRing(poly)

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

    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 AsText(ExteriorRing(GeomFromText(@poly)));
+-------------------------------------------+
| AsText(ExteriorRing(GeomFromText(@poly))) |
+-------------------------------------------+
| LINESTRING(0 0,0 3,3 3,3 0,0 0)           |
+-------------------------------------------+

  • InteriorRingN(poly,N)

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

    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 AsText(InteriorRingN(GeomFromText(@poly),1));
+----------------------------------------------+
| AsText(InteriorRingN(GeomFromText(@poly),1)) |
+----------------------------------------------+
| LINESTRING(1 1,1 2,2 2,2 1,1 1)              |
+----------------------------------------------+

  • NumInteriorRings(poly)

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

    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 NumInteriorRings(GeomFromText(@poly));
+---------------------------------------+
| NumInteriorRings(GeomFromText(@poly)) |
+---------------------------------------+
|                                     1 |
+---------------------------------------+

User Comments

Posted by [name withheld] on November 12 2006 6:48pm[Delete] [Edit]

there are more than just these functions available. I think this not clear enough in the docs.
for example, the MultiPolygon 'Centroid' function works for Polygons as well.
example:

mysql> SET @poly = 'Polygon((0 0,0 3,3 3,3 0,0 0))';

mysql> select astext( Centroid(PolygonFromText(@poly)));

+-------------------------------------------+
| astext( Centroid(PolygonFromText(@poly))) |
+-------------------------------------------+
| POINT(1.5 1.5)                            |
+-------------------------------------------+

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