ST_SnapToSelf

Signature

GEOMETRY ST_SnapToSelf(GEOMETRY geom, DOUBLE snapTolerance);
GEOMETRY ST_SnapToSelf(GEOMETRY geom, DOUBLE snapTolerance, BOOLEAN clean);

Description

Snap a geometry (geom) to itself with a given tolerance (snapTolerance).

Optionally, clean parameter allow to clean the resulting geometry making sure it is topologically valid. TRUE by default.

Snapping a geometry to itself can remove artifacts such as very narrow slivers, gores and spikes.

Examples

With `POLYGON`

SELECT ST_SnapToSelf('POLYGON ((1 1, 1 4, 5.5 4, 6 6, 5 1, 1 1))', 0.5);

-- Answer: POLYGON ((1 1, 1 4, 5.5 4, 5 1, 1 1))

With `LINESTRING`

Here snapTolerance is bigger than the distance between points [3 1] and [3.5 1]. So the resulting LINESTRING has a new point

SELECT ST_SnapToSelf('LINESTRING (1 1, 3 1, 5 4, 3.5 1, 6 1)', 1);

-- Answer: LINESTRING (1 1, 3 1, 5 4, 3 1, 3.5 1, 6 1)

Here snapTolerance is smaller than the distance between points [3 1] and [3.5 1]. So nothing changes.

SELECT ST_SnapToSelf('LINESTRING (1 1, 3 1, 5 4, 3.5 1, 6 1)', 0.2);

-- Answer: LINESTRING (1 1, 3 1, 5 4, 3.5 1, 6 1)

With `clean` parameter

`clean` is `TRUE`

SELECT ST_SnapToSelf('POLYGON ((1 1, 3.5 2.5, 1 4, 4 4, 4 2, 6 6, 6 1, 3.5 2, 1 1))', 
                     1, TRUE);

-- Answer: MULTIPOLYGON (((3.5 2, 4 4, 6 6, 6 1, 4 2, 3.5 2)), 
--                       ((3.5 2, 1 4, 4 4, 3.5 2.5, 3.5 2)))

`clean` is `FALSE`

SELECT ST_SnapToSelf('POLYGON ((1 1, 3.5 2.5, 1 4, 4 4, 4 2, 6 6, 6 1, 3.5 2, 1 1))', 
                     1, FALSE);

-- Answer: POLYGON ((1 1, 3.5 2, 1 4, 4 4, 3.5 2.5, 3.5 2, 4 4, 6 6, 6 1, 4 2, 3.5 2, 1 1))

See also