R = 6371000 # Radius of the Earth
S1 = Point(35.18028,136.90667) # Nagoya
S2 = Point(35.68944,139.69167) # Tokyo

# ground distance
function dG(Si::Point, Sj::Point)
  φi, λi = deg2rad(Si.φ), deg2rad(Si.λ)
  φj, λj = deg2rad(Sj.φ), deg2rad(Sj.λ)
  v1 = sin( (φj - φi) / 2 )
  v2 = sin( (λj - λi) / 2 )
  return 2 * R * asin(sqrt( v1^2 + cos(φi) * cos(φj) * v2^2 ))
end

println("Example dG = $(dG(S1, S2)/1000) [km]")