ST_Envelope

Signatures

GEOMETRY ST_Envelope(GEOMETRY geom);
GEOMETRY ST_Envelope(GEOMETRY geom, INT srid);
GEOMETRY ST_Envelope(GEOMETRYCOLLECTION geom);
GEOMETRY ST_Envelope(GEOMETRYCOLLECTION geom, INT srid);

Description

Returns the envelope of geom as a Geometry, optionally setting its SRID to srid. The default SRID is the same as that of geom.

Input type	Return type
A `POINT`	`POINT`
A line parallel to an axis	A two-vertex `LINESTRING`
Otherwise	A `POLYGON` whose coordinates are `(minx miny, maxx miny, maxx maxy, minx maxy, minx miny)`

Note

Implements the OpenGIS Simple Features Implementation Specification for SQL version 1.2.1

Examples

The envelope of a point is a point

SELECT ST_Envelope('POINT(1 2)', 2154);
-- Answer: POINT(1 2)

This is a line parallel to the x-axis, so only the endpoints are conserved

SELECT ST_Envelope('LINESTRING(1 1, 5 1, 9 1)');
-- Answer: LINESTRING(1 1, 9 1)

(minx miny, maxx miny, maxx maxy, minx maxy, minx miny)

SELECT ST_Envelope('MULTIPOINT(1 2, 3 1, 2 2, 5 1, 1 -1)');
-- Answer: POLYGON((1 -1, 1 2, 5 2, 5 -1, 1 -1))

(minx miny, maxx miny, maxx maxy, minx maxy, minx miny)

SELECT ST_Envelope('LINESTRING(1 2, 5 3, 2 6)');
-- Answer: POLYGON((1 2, 1 6, 5 6, 5 2, 1 2))

Setting or preserving the SRID

This shows that ST_Envelope preserves the SRID of the input geometry

SELECT ST_SRID(ST_Envelope(
    ST_GeomFromText('LINESTRING(1 1, 5 5)', 27572)))
-- Answer: 27572

This shows that ST_Envelope can set the SRID of Envelope

SELECT ST_SRID(ST_Envelope(
    ST_GeomFromText('LINESTRING(1 1, 5 5)', 27572), 2154))
-- Answer: 2154

Comparison with `ST_Extent`

CREATE TABLE input_table(geom GEOMETRY);
INSERT INTO input_table VALUES
     ('POLYGON((0 0, 3 -1, 1.5 2, 0 0))'),
     ('POLYGON((2 0, 3 3, 4 2, 2 0))'),
     ('POINT(5 6)'),
     ('LINESTRING(1 1, 1 6)');

ST_Envelope is a scalar function.

SELECT ST_Envelope(geom) ENV FROM input_table;

Answer:

ENV
POLYGON((0 -1, 0 6, 5 6, 5 -1, 0 -1))
POLYGON((2 0, 2 3, 4 3, 4 0, 2 0))
POINT(5 6)
LINESTRING(1 1, 1 6)

ST_Extent is an aggregate function.

SELECT ST_Extent(geom) EXT FROM input_table;

Answer: POLYGON((0 -1, 0 2, 3 2, 3 -1, 0 -1))

However, we can make ST_Envelope behave like ST_Extent as follows:

SELECT ST_Equals(ST_Envelope(ST_Accum(geom)),
                 ST_Extent(geom)) FROM input_table;

Answer: TRUE

Comparison with `ST_MinimumRectangle`

CREATE TABLE input_table(geom GEOMETRY);
INSERT INTO input_table VALUES
     ('POLYGON((0 0, 3 -1, 3 1, 2 1, 0 0))'),
     ('POLYGON((2 0, 3 3, 4 2, 2 0))'),
     ('POINT(5 6)'),
     ('LINESTRING(1 1, 1 6)');

SELECT ST_Envelope(geom) ENV, ST_MinimumRectangle(geom) MinRect
      FROM input_table;

Answer:

ENV	MinRect
POLYGON((0 -1, 0 1, 3 1, 3 -1, 0 -1))	POLYGON((3 -1, 3.59 0.80, 0.6 1.79, 0 -0, 3 -1))
POLYGON((2 0, 2 3, 4 3, 4 0, 2 0))	POLYGON((2 -0, 3.2 -0.40, 4.20 2.59, 3 2.99, 2 0))
POINT(5 6)	POINT(5 6)
LINESTRING(1 1, 1 6)	LINESTRING(1 1, 1 6)