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##### area (with strategy)

Calculates the area of a geometry using the specified strategy.

###### Description

The free function area calculates the area of a geometry using the specified strategy. Reasons to specify a strategy include: use another coordinate system for calculations; construct the strategy beforehand (e.g. with the radius of the Earth); select a strategy when there are more than one available for a calculation.

###### Synopsis

    template<typename Geometry, typename Strategy>
    auto area(Geometry const & geometry, Strategy const & strategy)

###### Parameters

Type | Concept | Name | Description
--- | --- | --- | ---
Geometry const & | Any type fulfilling a Geometry Concept | geometry | A model of the specified concept
Strategy const & | Any type fulfilling a Area Strategy Concept | strategy | The strategy which will be used for area calculations

###### Returns

The calculated area

###### Header

Either

`#include <boost/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 |
Variant |

###### Complexity

Linear

###### Available Strategies

- Cartesian
- Spherical
- Geographic

###### Example

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>

    namespace bg = boost::geometry;

    int main()
    {
        // Create spherical polygon
        bg::model::polygon<bg::model::point<double, 2, bg::cs::spherical_equatorial<bg::degree>>> sph_poly;
        bg::read_wkt("POLYGON((0 0,0 1,1 0,0 0))", sph_poly);

        // Create spherical strategy with mean Earth radius in meters
        bg::strategy::area::spherical<> sph_strategy(6371008.8);

        // Calculate the area of a spherical polygon
        double area = bg::area(sph_poly, sph_strategy);
        std::cout << "Area: " << area << std::endl;

        // Create geographic polygon
        bg::model::polygon<bg::model::point<double, 2, bg::cs::geographic<bg::degree>>> geo_poly;
        bg::read_wkt("POLYGON((0 0,0 1,1 0,0 0))", geo_poly);

        // Create geographic strategy with WGS84 spheroid
        bg::srs::spheroid<double> spheroid(6378137.0, 6356752.3142451793);
        bg::strategy::area::geographic<> geo_strategy(spheroid);

        // Calculate the area of a geographic polygon
        area = bg::area(geo_poly, geo_strategy);
        std::cout << "Area: " << area << std::endl;

        return 0;
    }

| Convenient namespace alias

Output:

    Area: 6.18249e+09
    Area: 6.15479e+09