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.

La fonction peut être appelée avec n'importe quel type de géométrie (y compris les GeometryCollections), mais seuls les lignes et polygones sont simplifiés. Les extrémités des lignes sont préservées.

	La géométrie renvoyée peut perdre sa simplicité (voir ST_IsSimple), la topologie peut ne pas être préservée et les résultats polygonaux peuvent être invalides (voir ST_IsValid). Utilisez ST_SimplifyPreserveTopology pour préserver la topologie et garantir la validité. ST_CoverageSimplify préserve également la topologie et la validité.

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.

Cette fonction ne préserve pas les limites partagées entre les polygones. Utilisez ST_CoverageSimplify si cela est nécessaire.

Cette fonction gère la 3D et la troisième dimension affectera le résultat.

Disponibilité : 2.2.0

Exemples

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

Simplification d'une ligne.

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

Simplification d'un polygone.

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

Voir aussi

ST_SetEffectiveArea, ST_Simplify, ST_SimplifyPreserveTopology, ST_CoverageSimplify, Topologie ST_Simplify