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>
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.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

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>

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