ST_Simplify

Description

Returns a "simplified" version of the given geometry using the Douglas-Peucker algorithm. Will actually do something only with (multi)lines and (multi)polygons but you can safely call it with any kind of geometry. Since simplification occurs on a object-by-object basis you can also feed a GeometryCollection to this function.

The "preserve collapsed" flag will retain objects that would otherwise be too small given the tolerance. For example, a 1m long line simplified with a 10m tolerance. If the preserve flag is given, the line will not disappear. This flag is useful for rendering engines, to avoid having large numbers of very small objects disappear from a map leaving surprising gaps.

	Note that returned geometry might lose its simplicity (see ST_IsSimple )

	Note topology may not be preserved and may result in invalid geometries. Use (see ST_SimplifyPreserveTopology ) to preserve topology.

Availability: 1.2.2

Examples

A circle simplified too much becomes a triangle, medium an octagon,

SELECT ST_Npoints(the_geom) AS np_before,
       ST_NPoints(ST_Simplify(the_geom,0.1)) AS np01_notbadcircle,
       ST_NPoints(ST_Simplify(the_geom,0.5)) AS np05_notquitecircle,
       ST_NPoints(ST_Simplify(the_geom,1)) AS np1_octagon,
       ST_NPoints(ST_Simplify(the_geom,10)) AS np10_triangle,
       (ST_Simplify(the_geom,100) is null) AS  np100_geometrygoesaway
  FROM
    (SELECT ST_Buffer('POINT(1 3)', 10,12) As the_geom) AS foo;

 np_before | np01_notbadcircle | np05_notquitecircle | np1_octagon | np10_triangle | np100_geometrygoesaway
-----------+-------------------+---------------------+-------------+---------------+------------------------
        49 |                33 |                  17 |           9 |             4 | t
      

See Also

ST_IsSimple , ST_SimplifyPreserveTopology , Topology ST_Simplify