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

Subject: Re: [geometry] Units of area (with strategy)
From: Fernando Pelliccioni (fpelliccioni_at_[hidden])
Date: 2014-04-16 10:59:28

Hi Barend,

Thanks for replying.
I will try passing the radius of the Earth in kilometers to the strategy
constructor.

Thanks.

On Mon, Apr 14, 2014 at 5:44 PM, Barend Gehrels <barend_at_[hidden]> wrote:

>  Hi Fernando,
>
>
> Fernando Pelliccioni wrote On 14-4-2014 20:36:
>
> Any comments?
>
>  Thanks!
>
>
> On Mon, Apr 7, 2014 at 10:28 AM, Fernando Pelliccioni <
> fpelliccioni_at_[hidden]> wrote:
>
>>  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.
>>
>
> You can convert this to the Earth-considered-as-a-perfect-sphere by
> passing the radius of the Earth in the strategy. This is more or less what
> the documentation says. If you pass it in Kilometers you will get square
> kilometers, if you pass miles you get miles back.
>
> However, the Earth is not a perfect sphere so the above is a rough
> approximation. At this moment we don't have anything better yet, because
> most of the Geographic Coordinate System is still in extensions and does
> not contain all functionality yet.
>
> Regards, Barend
>
>
>
>
>
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