CG_Simplify

Description

Simplifies a geometry using SFCGAL's simplification algorithm, which reduces the number of points or vertices while preserving the essential features of the geometry. This function preserves Z and M values during simplification.

The algorithm is based on constrained triangulation and uses the CGAL Polyline Simplification 2 library with additional handling to preserve Z and M coordinates. When topology is preserved and geometries intersect, Z and M values are interpolated at intersection points.

This function works well with 3D terrain-like geometries (2.5D) but is not designed for vertical surfaces like walls.

Availability: 3.6.0 - requires SFCGAL >= 2.1.0

This method needs SFCGAL backend.

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

This function supports M coordinates.

Parameters

Return Value

Returns a simplified geometry with preserved Z and M values.

Examples

-- Simplify a polygon with a threshold of 0.5
SELECT ST_AsText(CG_Simplify(ST_GeomFromText('POLYGON((0 0, 0 1, 0.1 1, 0.2 1, 0.3 1, 0.4 1, 0.5 1, 1 1, 1 0, 0 0))'), 0.5));

-- Simplify a 3D terrain geometry while preserving topology and Z values
SELECT ST_AsText(CG_Simplify(ST_GeomFromText('LINESTRING Z(0 0 0, 0 1 1, 0.1 1 1, 0.2 1 1, 0.3 1 1, 1 1 2)'), 0.2, true));

-- Simplify a geometry with both Z and M values
SELECT ST_AsText(CG_Simplify(ST_GeomFromText('LINESTRING ZM(0 0 0 1, 0 1 1 2, 0.1 1 1 3, 0.2 1 1 4, 0.3 1 1 5, 1 1 2 6)'), 0.2));

-- Simplify two geometry together preserving Z and M values, without topology
SELECT ST_AsText(CG_Simplify('GEOMETRYCOLLECTION ZM(LINESTRING ZM(-1 -1 3 4, 0 0 10 100, 1 1 20 200, 0 2 15 150, 0 5 30 300, 2 19 25 250, -4 20 15 150), POLYGON ZM((0 0 10 100, 1 1 20 200, 0 2 15 150, 0 5 30 300, 2 19 25 250, -4 20 15 150, 0 0 10 100)))', 2, false));

-- Simplify two geometry together preserving Z and M values, with topology
SELECT ST_AsText(CG_Simplify('GEOMETRYCOLLECTION ZM(LINESTRING ZM(-1 -1 3 4, 0 0 10 100, 1 1 20 200, 0 2 15 150, 0 5 30 300, 2 19 25 250, -4 20 15 150), POLYGON ZM((0 0 10 100, 1 1 20 200, 0 2 15 150, 0 5 30 300, 2 19 25 250, -4 20 15 150, 0 0 10 100)))', 2, true));
      

WITH depts_pds as (SELECT ST_GeomFromText('GEOMETRYCOLLECTION(
POLYGON((88.46 158.85,90.77 171.54,147.31 173.85,146.15 145,173.85 119.62,146.15 103.46,112.69 118.46,91.92 93.08,65.38 101.15,34.23 121.92,41.15 142.69,49.23 143.85,88.46 158.85)),
POLYGON((112.69 118.46,146.15 103.46,190 60.77,185.38 43.46,126.54 26.15,83.85 28.46,67.69 64.23,43.46 58.46,10 83.85,34.23 121.92,65.38 101.15,91.92 93.08,112.69 118.46)))
') as geom)
SELECT geom FROM depts_pds;
        

WITH depts_pds as (SELECT ST_GeomFromText('GEOMETRYCOLLECTION(
POLYGON((88.46 158.85,90.77 171.54,147.31 173.85,146.15 145,173.85 119.62,146.15 103.46,112.69 118.46,91.92 93.08,65.38 101.15,34.23 121.92,41.15 142.69,49.23 143.85,88.46 158.85)),
POLYGON((112.69 118.46,146.15 103.46,190 60.77,185.38 43.46,126.54 26.15,83.85 28.46,67.69 64.23,43.46 58.46,10 83.85,34.23 121.92,65.38 101.15,91.92 93.08,112.69 118.46)))
') as geom)
SELECT (ST_Dump(CG_Simplify(geom, 0.5, true))).geom FROM depts_pds;
        

WITH depts_pds as (SELECT ST_GeomFromText('GEOMETRYCOLLECTION(
POLYGON((88.46 158.85,90.77 171.54,147.31 173.85,146.15 145,173.85 119.62,146.15 103.46,112.69 118.46,91.92 93.08,65.38 101.15,34.23 121.92,41.15 142.69,49.23 143.85,88.46 158.85)),
POLYGON((112.69 118.46,146.15 103.46,190 60.77,185.38 43.46,126.54 26.15,83.85 28.46,67.69 64.23,43.46 58.46,10 83.85,34.23 121.92,65.38 101.15,91.92 93.08,112.69 118.46)))
') as geom)
SELECT (ST_Dump(CG_Simplify(geom, 0.5, false))).geom FROM depts_pds;
      

See Also

ST_Simplify , ST_SimplifyPreserveTopology