Name

ST_MakePolygon — Creates a Polygon formed by the given shell. Input geometries must be closed LINESTRINGS.

Synopsis

geometry ST_MakePolygon(geometry linestring);

geometry ST_MakePolygon(geometry outerlinestring, geometry[] interiorlinestrings);

Description

Creates a Polygon formed by the given shell. Input geometries must be closed LINESTRINGS. Comes in 2 variants.

Variant 1: Takes one closed linestring.

Variant 2: Creates a Polygon formed by the given shell and array of holes. You can construct a geometry array using the PostgreSQL array_agg, ARRAY[] and ARRAY() constructs. Input geometries must be closed LINESTRINGS.

[Note]

This function will not accept a MULTILINESTRING. Use ST_LineMerge or ST_Dump to generate line strings.

This function supports 3d and will not drop the z-index.

Examples: Single closed LINESTRING

--2d line
SELECT ST_MakePolygon(ST_GeomFromText('LINESTRING(75.15 29.53,77 29,77.6 29.5, 75.15 29.53)'));
--If linestring is not closed
--you can add the start point to close it
SELECT ST_MakePolygon(ST_AddPoint(foo.open_line, ST_StartPoint(foo.open_line)))
FROM (
SELECT ST_GeomFromText('LINESTRING(75.15 29.53,77 29,77.6 29.5)') As open_line) As foo;

--3d closed line
SELECT ST_MakePolygon(ST_GeomFromText('LINESTRING(75.15 29.53 1,77 29 1,77.6 29.5 1, 75.15 29.53 1)'));

st_asewkt
-----------
POLYGON((75.15 29.53 1,77 29 1,77.6 29.5 1,75.15 29.53 1))

--measured line --
SELECT ST_MakePolygon(ST_GeomFromText('LINESTRINGM(75.15 29.53 1,77 29 1,77.6 29.5 2, 75.15 29.53 2)'));

st_asewkt
----------
POLYGONM((75.15 29.53 1,77 29 1,77.6 29.5 2,75.15 29.53 2))
			  

Examples: Outer shell with inner shells

Build a donut with an ant hole

SELECT ST_MakePolygon(
		ST_ExteriorRing(ST_Buffer(foo.line,10)),
	ARRAY[ST_Translate(foo.line,1,1),
		ST_ExteriorRing(ST_Buffer(ST_MakePoint(20,20),1)) ]
	)
FROM
	(SELECT ST_ExteriorRing(ST_Buffer(ST_MakePoint(10,10),10,10))
		As line )
		As foo;
		

Build province boundaries with holes representing lakes in the province from a set of province polygons/multipolygons and water linestrings.

[Note]

The CASE construct is used because feeding a null array into ST_MakePolygon results in NULL.

[Note]

A left join is used to guarantee we get all provinces back even if they have no lakes.

	SELECT p.gid, p.province_name,
		CASE WHEN
			array_agg(w.the_geom) IS NULL THEN p.the_geom
		ELSE  ST_MakePolygon(ST_LineMerge(ST_Boundary(p.the_geom)), array_agg(w.the_geom)) END
	FROM
		provinces p LEFT JOIN waterlines w
			ON (ST_Within(w.the_geom, p.the_geom) AND ST_IsClosed(w.the_geom))
	GROUP BY p.gid, p.province_name, p.the_geom;

	--Same example above but utilizing a correlated subquery
	--and PostgreSQL built-in ARRAY() function that converts a row set to an array

	SELECT p.gid,  p.province_name, CASE WHEN
		EXISTS(SELECT w.the_geom
			FROM waterlines w
			WHERE ST_Within(w.the_geom, p.the_geom)
			AND ST_IsClosed(w.the_geom))
		THEN
		ST_MakePolygon(ST_LineMerge(ST_Boundary(p.the_geom)),
			ARRAY(SELECT w.the_geom
				FROM waterlines w
				WHERE ST_Within(w.the_geom, p.the_geom)
				AND ST_IsClosed(w.the_geom)))
		ELSE p.the_geom END As the_geom
	FROM
		provinces p;
			  

See Also

ST_Boundary, ST_AddPoint, ST_GeometryType, ST_IsClosed, ST_LineMerge, ST_BuildArea