radiusOfCurvature function
Radius of curvature of meridian at latitude phi in km.
Implementation
double radiusOfCurvature(double phi,
{double er = earthEr, double fl = earthFl}) {
final s = math.sin(phi);
final e2 = (2 - fl) * fl;
return er * (1 - e2) / math.pow(1 - e2 * s * s, 1.5);
}