area (with strategy)

Calculates the area of a geometry using the specified strategy.

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>
area_result<Geometry, Strategy>::type 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>

#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

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