Returns a simplified geometry using the Douglas-Peucker algorithm. Use this function to reduce vertex count for map rendering or storage optimization.
Syntax
geometry ST_Simplify(geometry geomA, float tolerance, boolean preserveCollapsed);Parameters
| Parameter | Type | Description |
|---|---|---|
geomA | geometry | The input geometry to simplify. |
tolerance | float | The simplification threshold. |
preserveCollapsed | boolean | When true, retains small geometries that would otherwise disappear because they are smaller than the tolerance value. |
Usage notes
ST_Simplify accepts any geometry type, including GeometryCollection. The function processes each element individually, so it works with collections. Simplification operations are supported on MultiLine objects, MultiPolygon objects, and MultiPoint objects.
ST_Simplify may return invalid geometry or cause a geometry to lose simplicity, because it does not guarantee topological correctness. To preserve topology, use ST_SimplifyPreserveTopology instead.
When to use preserveCollapsed
When a geometry's scale is much smaller than the tolerance value, simplification can collapse it and the geometry object may disappear. Set preserveCollapsed to true to retain these small geometries.
This is useful in rendering engines where eliminating small geometries leaves visible gaps on a map.
Examples
The following examples use a LINESTRING geometry and compare results across different tolerance values.
Compare simplification at tolerance 0.25 and 0.5:
SELECT g,
ST_Simplify(g, 0.25),
ST_Simplify(g, 0.5)
FROM (
SELECT 'LINESTRING(0 0,2 2,3 1,3.5 1.5,5 0,5.25 0.25,5.5 0)'::geometry AS g
) AS t;
Related functions
ST_SimplifyPreserveTopology — simplifies geometry while guaranteeing a valid result and preserving topology