ST_Centroid

Définition

La fonction ST_Centroid part d'un objet polygon ou multipolygon et renvoie le point correspondant au centre de l'enveloppe de la géométrie. Cela signifie que le centroïde se trouve à mi-chemin entre les étendues x et y minimales et maximales de la géométrie.

Syntaxe

Oracle et PostgreSQL

sde.st_centroid (polygon sde.st_geometry)
sde.st_centroid (multipolygon sde.st_geometry)

SQLite

st_centroid (polygon geometryblob)
st_centroid (multipolygon geometryblob)

Type de retour

ST_Point

Exemple

Le technicien SIG municipal souhaite afficher les objets multipolygon des emprises de bâtiments comme points isolés dans une illustration de la densité de construction. Les emprises de bâtiments sont stockées dans la table bfp qui a été créée et renseignée à l'aide des instructions affichées pour chaque base de données :

Oracle

--Create and populate table
CREATE TABLE bfp (
 building_id integer,
 footprint sde.st_geometry);

INSERT INTO bfp VALUES (
 1,
 sde.st_polygon ('polygon ((0 0, 0 10, 10 10, 10 0, 0 0))', 4326)
);

INSERT INTO bfp VALUES (
 2,
 sde.st_polygon ('polygon ((20 0, 30 20, 40 0, 20 0))', 4326)
);

INSERT INTO bfp VALUES (
 3,
 sde.st_polygon ('polygon ((20 30, 25 35, 30 30, 20 30))', 4326)
);

--The ST_Centroid function returns the centroid of each building footprint multipolygon. 
--The ST_AsText function converts each centroid point into a text representation recognized by the application.
SELECT building_id,
 sde.st_astext (sde.st_centroid (footprint)) Centroid
 FROM bfp;

BUILDING_ID                   Centroid

   1                     POINT  (5.00000000 5.00000000)
   2                     POINT  (30.00000000 10.00000000)
   3                     POINT  (25.00000000 32.50000000)

PostgreSQL

--Create and populate table
CREATE TABLE bfp (
 building_id serial,
 footprint sde.st_geometry);

INSERT INTO bfp (footprint) VALUES (
 sde.st_polygon ('polygon ((0 0, 0 10, 10 10, 10 0, 0 0))', 4326)
);

INSERT INTO bfp (footprint) VALUES (
 sde.st_polygon ('polygon ((20 0, 30 20, 40 0, 20 0))', 4326)
);

INSERT INTO bfp (footprint) VALUES (
 sde.st_polygon ('polygon ((20 30, 25 35, 30 30, 20 30))', 4326)
);

--The ST_Centroid function returns the centroid of each building footprint multipolygon. 
--The ST_AsText function converts each centroid point into a text representation recognized by the application.
SELECT building_id, sde.st_astext (sde.st_centroid (footprint)) 
 AS centroid
 FROM bfp;

building_id                  centroid

   1                     POINT  (5 5)
   2                     POINT  (30 10)
   3                     POINT  (25 33)

SQLite

--Create table, add geometry column, and populate table
CREATE TABLE bfp (
 building_id integer primary key autoincrement not null
);

SELECT AddGeometryColumn (
 NULL,
 'bfp',
 'footprint',
 4326,
 'polygon',
 'xy',
 'null'
);

INSERT INTO bfp (footprint) VALUES (
 st_polygon ('polygon ((0 0, 0 10, 10 10, 10 0, 0 0))', 4326)
);

INSERT INTO bfp (footprint) VALUES (
 st_polygon ('polygon ((20 0, 30 20, 40 0, 20 0))', 4326)
);

INSERT INTO bfp (footprint) VALUES (
 st_polygon ('polygon ((20 30, 25 35, 30 30, 20 30))', 4326)
);

--The ST_Centroid function returns the centroid of each building footprint multipolygon. 
--The ST_AsText function converts each centroid point into a text representation recognized by the application.
SELECT building_id, st_astext (st_centroid (footprint)) 
 AS "centroid"
 FROM bfp;

building_id              centroid

   1                     POINT  (5.00000000 5.00000000)
   2                     POINT  (30.00000000 10.0000000)
   3                     POINT  (25.00000000 32.5000000)