Categories:

Geospatial functions

ST_SIMPLIFY

Given an input GEOGRAPHY or GEOMETRY object that represents a Line or Polygon, returns a simpler approximation of the object. The function identifies and removes selected vertices, resulting in a similar object that has fewer vertices.

For example, if the input object is a Polygon with 50 vertices, ST_SIMPLIFY can return a simpler Polygon with only 20 of those vertices.

When simplifying an object, the function removes a vertex only if the distance between that vertex and the edge resulting from the removal of that vertex is within the specified tolerance.

Syntax

ST_SIMPLIFY( <geography_expression>, <tolerance> [ , <preserve_collapsed> ] )
ST_SIMPLIFY( <geometry_expression>, <tolerance> )
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Arguments

Required:

geography_expression . OR . geometry_expression

The GEOGRAPHY or GEOMETRY object to simplify.

Depending on the type of the GEOGRAPHY or GEOMETRY object, ST_SIMPLIFY has the following effect:

Type of Object

Effect of ST_SIMPLIFY

LineString, MultiLineString, Polygon, or MultiPolygon

ST_SIMPLIFY applies the simplification algorithm

Point or MultiPoint

ST_SIMPLIFY has no effect.

GeometryCollection or FeatureCollection

For GEOGRAPHY objects, ST_SIMPLIFY applies the simplification algorithm to each object in the collection. . . For GEOMETRY objects, ST_SIMPLIFY does not support these types.
tolerance

The maximum distance used by the simplification algorithm. Depending on the data type of the object, the following units are used for the operation:

  • GEOGRAPHY - Distance is interpreted in meters.

  • GEOMETRY - Distance is interpreted in the units of the object’s SRID (spatial reference system identifier). For example, the distance is interpreted in degrees for EPSG:4326, in meters for many projected SRIDs, or in feet for some local SRIDs.

    If the distance exceeds this tolerance for a candidate vertex, ST_SIMPLIFY keeps that vertex in the simplified object.

Optional:

preserve_collapsed

(For GEOGRAPHY objects only) If TRUE, retains objects that would otherwise be too small given the tolerance.

For example, when preserve_collapsed is FALSE and tolerance is 10 (meters), a 1m long line is reduced to a point in the simplified object. When preserve_collapsed is TRUE, the line is preserved in the simplified object.

Default: FALSE.

Returns

The function returns a value of type GEOGRAPHY or GEOMETRY.

Examples

GEOGRAPHY examples

The examples in this section display output in WKT format:

alter session set GEOGRAPHY_OUTPUT_FORMAT='WKT';
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The following example returns a simplified LineString that has fewer vertices than the original LineString. In the simplified object, a vertex is omitted if the distance between the vertex and the edge that replaces the vertex is less than 1000 meters.

SELECT ST_SIMPLIFY(
    TO_GEOGRAPHY('LINESTRING(-122.306067 37.55412, -122.32328 37.561801, -122.325879 37.586852)'),
    1000);
+----------------------------------------------------------------------------------------------------+
| ST_SIMPLIFY(                                                                                       |
|     TO_GEOGRAPHY('LINESTRING(-122.306067 37.55412, -122.32328 37.561801, -122.325879 37.586852)'), |
|     1000)                                                                                          |
|----------------------------------------------------------------------------------------------------|
| LINESTRING(-122.306067 37.55412,-122.325879 37.586852)                                             |
+----------------------------------------------------------------------------------------------------+
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GEOMETRY examples

The examples in this section display output in WKT format:

ALTER SESSION SET GEOMETRY_OUTPUT_FORMAT='WKT';
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The following example returns a simplified LineString that has fewer vertices than the original LineString. In the simplified object, a vertex is omitted if the distance between the vertex and the edge that replaces the vertex is less than 500 meters.

SELECT ST_SIMPIFY(
  TO_GEOMETRY('LINESTRING(1100 1100, 2500 2100, 3100 3100, 4900 1100, 3100 1900)'),
  500);

+----------------------------------------------------------------------------------------------------+
| ST_SIMPLIFY(TO_GEOMETRY('LINESTRING(1100 1100, 2500 2100, 3100 3100, 4900 1100, 3100 1900)'), 500) |
|----------------------------------------------------------------------------------------------------|
| LINESTRING(1100 1100,3100 3100,4900 1100,3100 1900)                                                |
+----------------------------------------------------------------------------------------------------+
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The following example simplifies an ellipse that has 36 initial vertices to a shape with 16 or 10 vertices, depending on the tolerance argument:

SELECT ST_NUMPOINTS(geom) AS numpoints_before,
  ST_NUMPOINTS(ST_Simplify(geom, 0.5)) AS numpoints_simplified_05,
  ST_NUMPOINTS(ST_Simplify(geom, 1)) AS numpoints_simplified_1
  FROM
  (SELECT ST_BUFFER(to_geometry('LINESTRING(0 0, 1 1)'), 10) As geom);

+------------------+-------------------------+------------------------+
| NUMPOINTS_BEFORE | NUMPOINTS_SIMPLIFIED_05 | NUMPOINTS_SIMPLIFIED_1 |
|------------------+-------------------------+------------------------|
|               36 |                      16 |                     10 |
+------------------+-------------------------+------------------------+
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