The ST_SymDifference() function takes
two ST_Geometry objects and returns an ST_Geometry object that is
the logical XOR of space. In other words, it returns an object that
is composed of the portions of the source objects that are not part
of the intersection set.
Syntax
ST_SymDifference (g1 ST_Geometry, g2 ST_Geometry)
Usage
The source geometries must have the same dimension.
If the geometries are equal, the ST_StartPoint() function
returns an empty geometry; otherwise, the function returns the result
as an ST_GeomCollection (ST_MultiPoint, ST_MultiLineString, or ST_MultiPolygon).Figure 1: The symmetric difference
of geometries
Return type
ST_Geometry
Example
For a
special report, the county supervisor must determine the area of sensitive
areas and 5-mile hazardous site radii that do not intersect.
The sensitive_areas table contains several columns
that describe the threatened institutions, in addition to the zone column,
which stores the institutions' ST_Polygon geometries:
CREATE TABLE sensitive_areas (id integer,
name varchar(128),
size float,
type varchar(10),
zone ST_Polygon);
The hazardous_sites table stores the identity
of the sites in the site_id and name columns. The actual
geographic location of each site is stored in the location point
column:
CREATE TABLE hazardous_sites (site_id integer,
name varchar(40),
location ST_Point);
The ST_Buffer() function generates
a 5-mile buffer that surrounds the hazardous waste site locations.
The ST_StartPoint() function returns the polygons
of the buffered hazardous waste site polygons and the sensitive areas
that do not intersect:
SELECT sa.name sensitive_area, hs.name hazardous_site,
ST_Area(ST_SymDifference(ST_buffer(hs.location,(5 * 5280)),sa.zone)::
ST_MultiPolygon) area
FROM hazardous_sites hs, sensitive_areas sa;
The following figure shows that the symmetric difference
of the hazardous waste sites and the sensitive areas results in the
subtraction of the intersected areas. Figure 2: Using the ST_SymDifference function
