GIS::Distance::GreatCircle - Great circle distance calculations.
A true Great Circle Distance calculation. This was created because the GIS::Distance::MathTrig formula uses Math::Trig's
great_circle_distance() which doesn't actually appear to use the actual Great Circle Distance formula (more likely Cosine).
A faster (XS) version of this formula is available as GIS::Distance::Fast::GreatCircle.
Normally this module is not used directly. Instead GIS::Distance is used which in turn interfaces with the various formula classes.
c = 2 * asin( sqrt( ( sin(( lat1 - lat2 )/2) )**2 + cos( lat1 ) * cos( lat2 ) * ( sin(( lon1 - lon2 )/2) )**2 ) )