Geometry Boost List: [geometry] Units of area (with strategy)

Subject: [geometry] Units of area (with strategy)
From: Fernando Pelliccioni (fpelliccioni_at_[hidden])
Date: 2014-04-07 09:28:16

Hi all,

I run the "area (with strategy)" example shown on

http://www.boost.org/doc/libs/1_55_0/libs/geometry/doc/html/geometry/reference/algorithms/area/area_2_with_strategy.html

I'm interested in getting the geodetic area (the second one of the code
below)

#include <iostream>
#include <boost/geometry.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 spherical polygon (for latitude: 0 at
equator)
    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);
    double area = bg::area(sph_poly);
    std::cout << "Area: " << area << std::endl;

    return 0;
}

The code above prints out:
Area: 0.339837

In this case, what is the unit of the area?

The documentation states that:
Returns the area on a unit sphere (or another sphere, if specified as such
in the constructor of the strategy)

Sorry, but my knowledge of geography is limited.
How can I transform that number to square kilometers, square miles, ...?

Thank you very much.

Regards,
Fernando Pelliccioni.