The ST_Area() function

The ST_Area() function returns the area of a polygon or multipolygon.

Syntax

ST_Area(pl1 ST_Polygon)
ST_Area(pl1 ST_Polygon, linear_uom varchar(128))

ST_Area(mpl1 ST_MultiPolygon)
ST_Area(mpl1 ST_MultiPolygon, linear_uom varchar(128))

ST_Area(pl1 ST_Polygon)
ST_Area(mpl1 ST_MultiPolygon)

The linear_uom parameter converts the result to the specified unit of measure. To calculate the area if the polygon is in a geographic coordinate system where the coordinates are in an angular unit of measure, you must specify a linear unit of measure with the linear_uom parameter. Angular units of measure are converted to linear units of measure by great-circle calculations. If the polygon is in a projected coordinate system that has a unit of measure that is different from the unit of measure that is specified by the linear_uom parameter, then the returned value is converted to the unit of measure that is specified by the linear_uom parameter. The linear_uom parameter must be the name of a linear unit of measure from the unit_name column of the st_units_of_measure table.

Return type

DOUBLE PRECISION

Example: Find the area of buildings

The city engineer needs a list of building areas. To create the list, a GIS technician selects the building ID and area of each building footprint.

The building footprints are stored in the buildingfootprints table that is created with the following CREATE TABLE statement:

CREATE TABLE buildingfootprints (building_id  integer,
                                 lot_id       integer,
                                 footprint    ST_MultiPolygon);

To satisfy the city engineer's request, the technician selects the unique key, the building_id, and the area of each building footprint from the buildingfootprints table:

SELECT building_id, ST_Area(footprint) area
   FROM buildingfootprints;

building_id           area 

        506 78.00000000000
        543 68.00000000000
       1208 123.0000000000
        178 87.00000000000

The following figure shows the four building footprints that are labeled with their building ID numbers and displayed alongside their adjacent street.

The top left building is number 506. The top right building is number 543. The lower left building in number 1208. The lower right building is number 178. — Figure 1: Building footprints

Examples: Find the areas of polygons

The following statement returns the area of a polygon in square meters:

EXECUTE FUNCTION round(
        st_area(
                '32608 polygon((576100 15230, 576100 15232, 576102 15232, 
                 576102 15230, 576100 15230))'::st_polygon,
                'meter'),
        2);

    (expression)

4.00000000000000

1 row(s) retrieved.

The following statement returns the area of a multipolygon in square meters:

EXECUTE FUNCTION round(
        st_area(
                '32608 multipolygon(((576100 15230, 576100 15232, 576102 15232, 
                 576102 15230, 576100 15230)),((576104 4, 576104 6, 576106 6, 
                 576106 4, 576104 4)))'::st_multipolygon,
                'meter'),
        2);

    (expression)

8.00000000000000

1 row(s) retrieved.

Example: Find the area of a polygon that has angular coordinates

The following statement returns the area in square meters of a 10 kilometer buffer around the coordinates from the angular coordinate system WGS 84 - 4326 that represent the latitude and longitude of New York (40.67000 N, 73.94000 W):

EXECUTE FUNCTION ST_Area(ST_Buffer('4326 point(-73.94000 40.67000)'
                ::st_point, 10, 'kilometer')::st_polygon, 'meter');

  (expression)

313934956.2857