# Geometry :

Subject: Re: [geometry] spherical_equitorial to cartesian transform confusion
From: Barend Gehrels (barend_at_[hidden])
Date: 2012-07-31 17:32:22

Hi Joe,

On 30-7-2012 3:37, joe_at_[hidden] wrote:
> Most of the time things in my model are spherical_equatorial with
> altitude,
> using degrees. But it would be cool to be able to transform arbitrary
> things in
> my model into 3d cartesian coordinates, for example to use GGL to
> compute the
> straight line distance between points rather than do the trig myself.
>
> Well I don't understand how the provided transform stuff is supposed
> to work. I
> get the "WTF" seen below with boost 1.50.0. If I convert various
> spherical_equatorial points to cartesian 'distance' always returns
> only the
> change in altitude.
>
> Is what I'm trying to do actually implemented?
>
> ----------------------------------------
> class MyLL { double lat, lng, alt; ... }
>
> BOOST_GEOMETRY_REGISTER_POINT_3D_GET_SET(MyLL, double,
> bg::cs::spherical_equatorial<bg::degree>, get_lng, get_lat, get_alt,
> set_lng, set_lat, set_alt)
>
> MyLL amsterdam(52.37, 4.90, 0);
>
> bg::model::point<double, 3, bg::cs::cartesian> cartesian_amsterdam;
> bg::transform(amsterdam, cartesian_amsterdam);
>
> std::cout << bg::dsv(amsterdam) << std::endl; // (4.9, 52.37, 0)
> std::cout << bg::dsv(cartesian_amsterdam) << std::endl; // WTF: (0, 0, 0)
>
> MyLL paris(48.86, 2.35, 0);
> bg::model::point<double, 3, bg::cs::cartesian> cartesian_paris;
> bg::transform(paris, cartesian_paris);
>
> /* I expect this to return a straight line distance, a la law of
> cosines. */
> bg::distance(cartesian_amsterdam, cartesian_paris); )
> /* doesn't work because transform doesn't work like I thought, but
> ultimately I
> * expect the result should be:
> * double gamma = bg::distance(paris, amsterdam);
> * double alen = EarthRadius + paris.get_alt();
> * double blen = EarthRadius + amsterdam.get_alt();
> * double distance = sqrt( alen*alen + blen*blen - (2*alen*blen *
> std::cos(gamma)) );
> */
>
> __

I cannot yet say what is going wrong.
But I can confirm that what you describe in the commented expectactions
is indeed not implemented.

I have to come back on this later.

Regards, Barend

Geometry list run by mateusz at loskot.net