Bad value of EARTH_RADIUS

[cffk]

EARTH_RADIUS is earth's radius for use in great circle calculations where the earth is treated as a sphere. The recommended value for such a quantity is the mean radius = (2a + b)/3 = 6371 km. The value defined in coordinate_calculation.hpp and haversine.hpp is about (3a + b)/4 = 6372.8 km. There's no justification for this value.

[danpat]

@cffk Ultimately, the 6372.8km value comes from http://math.wikia.com/wiki/Ellipsoidal_quadratic_mean_radius. I believe it was copied from libosmium

Whether it's the right thing to use or not is another question. We have a PR open at the moment here #5041 that will likely deprecate this constant.

[cffk]

Yes, I know about this source. Unfortunately, I can't make head or tail of it. There's a long discussion between me and Kaimbridge, the proponent of the quadratic mean radius here. Click on the "show" for the "extended discussion".