Description

Returns a simplified representation of a geometry using the Visvalingam-Whyatt algorithm. The simplification tolerance is an area value, in squared units of the input SRS. Simplification removes vertices which form "corners" with area less than the tolerance. The result may not be valid even if the input is.

The function can be called with any kind of geometry (including GeometryCollections), but only line and polygon elements are simplified. Endpoints of linear geometry are preserved.

	The returned geometry may lose its simplicity (see ST_IsSimple), topology may not be preserved, and polygonal results may be invalid (see ST_IsValid). Use ST_SimplifyPreserveTopology to preserve topology and ensure validity. ST_CoverageSimplify also preserves topology and validity.

This is a general-purpose simplification function. It may remove narrow polygon spikes when their effective area is below the tolerance, but it is not a dedicated spike-removal or polygon-repair operation. Use ST_IsValid to check the result and ST_MakeValid to repair invalid polygonal output if needed.

This function does not preserve boundaries shared between polygons. Use ST_CoverageSimplify if this is required.

This function handles 3D and the third dimension will affect the result.

Availability: 2.2.0

Examples

This polygon has a narrow spike whose effective area is small enough to be removed by a tolerance of 20 square units.

SELECT geom AS original, ST_SimplifyVW(geom, 20) AS simplified
FROM (SELECT 'POLYGON((0 0,10 0,10 10,5.2 10,5 16,4.8 10,0 10,0 0))'::geometry AS geom) AS t;

POLYGON((0 0,10 0,10 10,5.2 10,5 16,4.8 10,0 10,0 0)) | POLYGON((0 0,10 0,10 10,0 10,0 0))

01030000000100000008000000000000000000000000000000000000000000000000002440000000000000000000000000000024400000000000002440CDCCCCCCCCCC1440000000000000244000000000000014400000000000003040333333333333134000000000000024400000000000000000000000000000244000000000000000000000000000000000 | 010300000001000000050000000000000000000000000000000000000000000000000024400000000000000000000000000000244000000000000024400000000000000000000000000000244000000000000000000000000000000000

Figure

A LineString is simplified with a minimum-area tolerance of 30. For a linear simplification using ST_Simplify, a comparable tolerance would be a length in input-SRS units, so the value here is roughly that linear tolerance squared.

SELECT ST_SimplifyVW(geom, 30) simplified
  FROM (SELECT 'LINESTRING(5 2,3 8,6 20,7 25,10 10)'::geometry AS geom) AS t;

LINESTRING(5 2,7 25,10 10)

010200000003000000000000000000144000000000000000400000000000001C40000000000000394000000000000024400000000000002440

Figure

Simplifying a line.

SELECT ST_SimplifyVW('LINESTRING (10 10,50 40,30 70,50 60,70 80,50 110,100 100,90 140,100 180,150 170,170 140,190 90,180 40,110 40,150 20)'::geometry,
1600);

LINESTRING(10 10,100 100,100 180,150 170,190 90,150 20)

0102000000060000000000000000002440000000000000244000000000000059400000000000005940000000000000594000000000008066400000000000C0624000000000004065400000000000C0674000000000008056400000000000C062400000000000003440

Figure

Simplifying a polygon.

SELECT ST_SimplifyVW('POLYGON ((20 20,50 60,20 120,40 170,80 180,120 140,180 180,160 130,180 100,180 30,130 70,150 10,20 20))'::geometry,
1600);

POLYGON((20 20,20 120,80 180,160 130,180 30,20 20))

010300000001000000060000000000000000003440000000000000344000000000000034400000000000005E40000000000000544000000000008066400000000000006440000000000040604000000000008066400000000000003E4000000000000034400000000000003440

Figure

See Also

ST_SetEffectiveArea, ST_Simplify, ST_SimplifyPreserveTopology, ST_CoverageSimplify, Topology ST_Simplify