ST_Force2D

Signature

GEOMETRY ST_Force2D(GEOMETRY geom);

Converts a 3D or 4D Geometry to a 2D Geometry by deleting the z-value or the m-value of each coordinate if it exists.

SELECT ST_Force2D('POINT Z(-10 10 6)');
-- Answer:         POINT(-10 10)

SELECT ST_Force2D('LINESTRING Z(-10 10 2, 10 10 3)');
-- Answer:         LINESTRING(-10 10, 10 10)

SELECT ST_Force2D('POLYGON M((2 2 2, 10 0 1, 10 5 1, 0 5 2, 2 2 2))');
-- Answer:         POLYGON((2 2, 10 0, 10 5, 0 5, 2 2))

SELECT ST_Force2D('POLYGON ZM((2 2 3 2, 10 0 1 1, 10 5 2 1, 0 5 2 2, 2 2 3 2))');
-- Answer:         POLYGON((2 2, 10 0, 10 5, 0 5, 2 2))

No effect on 2D Geometries:

SELECT ST_Force2D('POINT(-10 10)');
-- Answer:         POINT(-10 10)

Not working with Geometries having mixed dimension:

SELECT ST_Force2D('LINESTRING(-10 10, 10 10 3)');
-- Answer:         Data conversion error converting "LINESTRING(-10 10, 10 10 3)"