## Name

ST_ConvexHull — Computes the convex hull of a geometry.

## Synopsis

`geometry `**ST_ConvexHull**(

geometry `geomA``)`

;

## Description

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. s2.1.1.3

This method implements the SQL/MM specification. SQL-MM 3: 5.1.16

This function supports 3d and will not drop the z-index.

## Examples

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;