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##### area

Calculates the area of a geometry.

###### Description

The free function area calculates the area of a geometry. It uses the default strategy, based on the coordinate system of the geometry.

The area algorithm calculates the surface area of all geometries having a surface, namely box, polygon, ring, multipolygon. The units are the square of the units used for the points defining the surface. If subject geometry is defined in meters, then area is calculated in square meters.

The area calculation can be done in all three common coordinate systems, Cartesian, Spherical and Geographic as well.

###### Synopsis

    template<typename Geometry>
    default_area_result<Geometry>::type area(Geometry const & geometry)

###### Parameters

Type | Concept | Name | Description
--- | --- | --- | ---
Geometry const & | Any type fulfilling a Geometry Concept | geometry | A model of the specified concept

###### Returns

The calculated area

###### Header

Either

`#include <boost/geometry/geometry.hpp>`

Or

`#include <boost/geometry/algorithms/area.hpp>`

###### Conformance

The function area implements function Area from the OGC Simple Feature Specification.

###### Behavior

Case | Behavior
--- | ---
pointlike (e.g. point) | Returns 0
linear (e.g. linestring) | Returns 0
areal (e.g. polygon) | Returns the area
Cartesian | Returns the area in the same units as the input coordinates
Spherical | Returns the area on a unit sphere (or another sphere, if specified as such in the constructor of the strategy)
Reversed polygon (coordinates not according their orientiation) | Returns the negative area

###### Supported geometries

Geometry | Status
--- | ---
Point |
Segment |
Box |
Linestring |
Ring |
Polygon |
MultiPoint |
MultiLinestring |
MultiPolygon |

###### Complexity

Linear

###### Examples

Calculate the area of a polygon

    #include <iostream>

    #include <boost/geometry.hpp>
    #include <boost/geometry/geometries/point_xy.hpp>
    #include <boost/geometry/geometries/polygon.hpp>
    #include <boost/geometry/io/wkt/wkt.hpp>

    namespace bg = boost::geometry;

    int main()
    {
        // Calculate the area of a cartesian polygon
        bg::model::polygon<bg::model::d2::point_xy<double> > poly;
        bg::read_wkt("POLYGON((0 0,0 7,4 2,2 0,0 0))", poly);
        double area = bg::area(poly);
        std::cout << "Area: " << area << std::endl;

        // Calculate the area of a spherical equatorial polygon
        bg::model::polygon<bg::model::point<float, 2, bg::cs::spherical_equatorial<bg::degree> > > sph_poly;
        bg::read_wkt("POLYGON((0 0,0 45,45 0,0 0))", sph_poly);
        area = bg::area(sph_poly);
        std::cout << "Area: " << area << std::endl;

        return 0;
    }

| Convenient namespace alias

Output:

    Area: 16
    Area: 0.339837