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 in meters between a vertex and the edge resulting from the removal of the vertex. If the distance exceeds this tolerance for a 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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