Gets the distance between two latitude and longitude points.

GeoCoordinate.cs

// http://www.geodatasource.com/developers/c-sharp

using System;

namespace <your_namespace>
{
    public class GeoCoordinate
    {
        public double Latitude { get; set; }

        public double Longitude { get; set; }

        public GeoCoordinate(double latitude, double longitude)
        {
            Latitude = latitude;
            Longitude = longitude;
        }

        public double GetDistanceTo(GeoCoordinate coordinate)
        {
            return Distance(Latitude, Longitude, coordinate.Latitude, coordinate.Longitude, 'K');
        }

        public double GetDistanceToInKm(GeoCoordinate coordinate)
        {
            return Distance(Latitude, Longitude, coordinate.Latitude, coordinate.Longitude, 'K');
        }

        public double GetDistanceToInNauticalMiles(GeoCoordinate coordinate)
        {
            return Distance(Latitude, Longitude, coordinate.Latitude, coordinate.Longitude, 'N');
        }

        private static double Distance(double lat1, double lon1, double lat2, double lon2, char unit)
        {
            double theta = lon1 - lon2;
            double dist = Math.Sin(Deg2Rad(lat1)) * Math.Sin(Deg2Rad(lat2)) + Math.Cos(Deg2Rad(lat1)) * Math.Cos(Deg2Rad(lat2)) * Math.Cos(Deg2Rad(theta));
            dist = Math.Acos(dist);
            dist = Rad2Deg(dist);
            dist = dist * 60 * 1.1515;
            if (unit == 'K')
            {
                dist = dist * 1.609344;
            }
            else if (unit == 'N')
            {
                dist = dist * 0.8684;
            }
            return (dist);
        }

        private static double Deg2Rad(double deg)
        {
            return (deg * Math.PI / 180.0);
        }

        private static double Rad2Deg(double rad)
        {
            return (rad / Math.PI * 180.0);
        }
    }
}