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ST_PointOnSurface

Returns a point that is guaranteed to lie on the surface of a geometry.

Syntax​

ST_PointOnSurface(geometry)

Parameters​

ParameterTypeDescription
geometryGeometryInput geometry

Returns​

TypeDescription
Binary (WKB)Point on surface as WKB with geoarrow.wkb metadata

Examples​

Point on Polygon​

SELECT ST_PointOnSurface(ST_GeomFromText('POLYGON((0 0, 4 0, 4 4, 0 4, 0 0))'));
-- Returns a point inside the polygon (e.g., POINT(2 2))

Point on Concave Polygon​

SELECT ST_PointOnSurface(ST_GeomFromText('POLYGON((0 0, 3 0, 3 1, 1 1, 1 3, 0 3, 0 0))'));
-- Returns a point guaranteed inside the L-shape

Label Placement​

SELECT name, ST_PointOnSurface(geom) as label_point
FROM regions;

Compare with Centroid​

SELECT
    ST_Centroid(geom) as centroid,
    ST_PointOnSurface(geom) as point_on_surface,
    ST_Contains(geom, ST_Centroid(geom)) as centroid_inside,
    ST_Contains(geom, ST_PointOnSurface(geom)) as pos_inside
FROM concave_polygons;

Notes​

  • Unlike ST_Centroid, always returns a point inside the geometry
  • Essential for concave polygons where centroid may be outside
  • Useful for label placement and representative point selection
  • For Points, returns the same point
  • For LineStrings, returns a point on the line

See Also​