ST_MakePolygon
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## Name

ST_MakePolygon — Creates a Polygon from a shell and optional list of holes.

## Synopsis

`geometry ST_MakePolygon(`geometry linestring`)`;

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

## Description

Creates a Polygon formed by the given shell and optional array of holes. Input geometries must be closed LineStrings (rings).

Variant 1: Accepts one shell LineString.

Variant 2: Accepts a shell LineString and an array of inner (hole) LineStrings. A geometry array can be constructed using the PostgreSQL array_agg(), ARRAY[] or ARRAY() constructs.

This function does not accept MultiLineStrings. Use ST_LineMerge to generate a LineString, or ST_Dump to extract LineStrings.

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

## Examples: Single input variant

Create a Polygon from a 2D LineString.

    SELECT ST_MakePolygon( ST_GeomFromText('LINESTRING(75 29,77 29,77 29, 75 29)'));

Create a Polygon from an open LineString, using ST_StartPoint and ST_AddPoint to close it.

    SELECT ST_MakePolygon( ST_AddPoint(foo.open_line, ST_StartPoint(foo.open_line)) )
    FROM (
      SELECT ST_GeomFromText('LINESTRING(75 29,77 29,77 29, 75 29)') As open_line) As foo;

Create a Polygon from a 3D LineString

    SELECT ST_AsEWKT( ST_MakePolygon( '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))

Create a Polygon from a LineString with measures

    SELECT ST_AsEWKT( ST_MakePolygon( '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 holes variant

Create a donut Polygon with an extra hole

    SELECT ST_MakePolygon( ST_ExteriorRing( ST_Buffer(ring.line,10)),
    	ARRAY[  ST_Translate(ring.line, 1, 1),
    		ST_ExteriorRing(ST_Buffer(ST_Point(20,20),1)) ]
    	)
    FROM (SELECT ST_ExteriorRing(
    	ST_Buffer(ST_Point(10,10),10,10)) AS line ) AS ring;

Create a set of province boundaries with holes representing lakes. The input is a table of province Polygons/MultiPolygons and a table of water linestrings. Lines forming lakes are determined by using ST_IsClosed. The province linework is extracted by using ST_Boundary. As required by `ST_MakePolygon`, the boundary is forced to be a single LineString by using ST_LineMerge. (However, note that if a province has more than one region or has islands this will produce an invalid polygon.) Using a LEFT JOIN ensures all provinces are included even if they have no lakes.

The CASE construct is used because passing a null array into ST_MakePolygon results in a NULL return value.

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

Another technique is to utilize a correlated subquery and the ARRAY() constructor that converts a row set to an array.

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

## See Also

ST_BuildArea ST_Polygon

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