private static int transit(double lon1, double lon2)
{
// Return 1 or -1 if crossing prime meridian in east or west direction.
// Otherwise return zero.
// Compute lon12 the same way as Geodesic.Inverse.
lon1 = GeoMath.AngNormalize(lon1);
lon2 = GeoMath.AngNormalize(lon2);
double lon12 = GeoMath.AngDiff(lon1, lon2);
int cross =
lon1 < 0 && lon2 >= 0 && lon12 > 0 ? 1 :
(lon2 < 0 && lon1 >= 0 && lon12 < 0 ? -1 : 0);
return(cross);
}