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##### length (with strategy)

Calculates the length of a geometry using the specified strategy.

###### Description

The free function length calculates the length (the sum of distances between consecutive points) 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>
    default_length_result< Geometry >::type length(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 distance Strategy Concept | strategy | The strategy which will be used for distance calculations

###### Returns

The calculated length

###### Header

Either

`#include <boost/geometry.hpp>`

Or

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

###### Conformance

The function length implements function Length from the OGC Simple Feature Specification.

###### Behavior

Case | Behavior
--- | ---
pointlike (e.g. point) | Returns 0
linear (e.g. linestring) | Returns the length
areal (e.g. polygon) | Returns 0

###### Complexity

Linear

###### Examples

The following example shows the length measured over a sphere, expressed in kilometers. To do that the radius of the sphere must be specified in the constructor of the strategy.

    #include <iostream>
    #include <boost/geometry.hpp>
    #include <boost/geometry/geometries/linestring.hpp>

    int main()
    {
        using namespace boost::geometry;
        using P = model::point<float, 2, cs::spherical_equatorial<degree>>;
        model::linestring<P> line;
        line.push_back(P(2, 41));
        line.push_back(P(2, 48));
        line.push_back(P(5, 52));
        double const mean_radius = 6371.0;
        std::cout << "length is "
            << length(line, strategy::distance::haversine<float>(mean_radius) )
            << " kilometers " << std::endl;

        return 0;
    }

| Wiki

Output:

    length is 1272.03 kilometers