using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        double lat1 = 16.0269642;

        double lat2 = 16.066757202148438;

        double lng1 = 108.2102712;

        double lng2 = 108.21389007568359;

        var result = DistanceTo(lat1, lng1, lat2, lng2);

        var result2 = distanceBetween2Points(lat1, lng1, lat2, lng2);

        //var result2 = distanceBetween2Points(lat1, lat2, lng1, lng2);

        Console.WriteLine(result);

        Console.WriteLine(result2);

    }

    public static double DistanceTo(double lat1, double lon1, double lat2, double lon2, char unit = 'K')

    {

        double rlat1 = Math.PI*lat1/180;

        double rlat2 = Math.PI*lat2/180;

        double theta = lon1 - lon2;

        double rtheta = Math.PI*theta/180;

        double dist =

            Math.Sin(rlat1)*Math.Sin(rlat2) + Math.Cos(rlat1)*

            Math.Cos(rlat2)*Math.Cos(rtheta);

        dist = Math.Acos(dist);

        dist = dist*180/Math.PI;

        dist = dist*60*1.1515;

​

        switch (unit)

        {

            case 'K': //Kilometers -> default

                return Math.Round(dist*1.609344,3);

            case 'N': //Nautical Miles

                return dist*0.8684;

            case 'M': //Miles

                return dist;

        }

​

        return dist;

    }

    public static double distanceBetween2Points(double la1, double lo1, double la2, double lo2) {

        double R = 6371;

        double dLat = (la2 - la1) * (Math.PI / 180);

        double dLon = (lo2 - lo1) * (Math.PI / 180);

        double la1ToRad = la1 * (Math.PI / 180);

        double la2ToRad = la2 * (Math.PI / 180);

        double a = Math.Sin(dLat / 2) * Math.Sin(dLat / 2) + Math.Cos(la1ToRad)

                    * Math.Cos(la2ToRad) * Math.Sin(dLon / 2) * Math.Sin(dLon / 2);

        double c = 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1 - a));

        double d = R * c;

        return Math.Round(d,3);

    }

}