ST_ConvexHull — Computes the convex hull of a geometry.
geometry ST_ConvexHull(geometry geomA
Computes the convex hull of a geometry. The convex hull is the smallest convex geometry that encloses all geometries in the input.
In the general case the convex hull is a Polygon. The convext hull of two or more collinear points is a two-point LineString. The convex hull of one or more identical points is a Point.
It is usually used with Multi* and GeometryCollections. It is not an aggregate function. To compute the convex hull of a set of geometries, use ST_Collect to aggregate them.
One can think of the convex hull as the geometry obtained by wrapping an elastic band around a set of geometries. This is different from a concave hull which is analogous to "shrink-wrapping" the geometries. A convex hull is often used to determine an affected area based on a set of point observations.
Performed by the GEOS module
This method implements the OpenGIS Simple Features Implementation Specification for SQL 1.1. s184.108.40.206
This method implements the SQL/MM specification. SQL-MM 3: 5.1.16
This function supports 3d and will not drop the z-index.
SELECT ST_AsText(ST_ConvexHull( ST_Collect( ST_GeomFromText('MULTILINESTRING((100 190,10 8),(150 10, 20 30))'), ST_GeomFromText('MULTIPOINT(50 5, 150 30, 50 10, 10 10)') )) ); ---st_astext-- POLYGON((50 5,10 8,10 10,100 190,150 30,150 10,50 5))
Using with ST_Collect to compute the convex hulls of geometry sets.
--Get estimate of infected area based on point observations SELECT d.disease_type, ST_ConvexHull(ST_Collect(d.the_geom)) As the_geom FROM disease_obs As d GROUP BY d.disease_type;