CalcDistance

GeoDistance.cs

// Calcluate airline distance based on mercator distance.
// This approximation formula is sufficiently accurate for 
// our needs for distances of up to 600 km and 
// 80° latitude (the error is never more than 5% even for 
// extreme values). 
using System;

public class Program {
    public static void Main () {
        double lat1 = 49.0;
        double lon1 = 8.0;
      
        double lat2 = 50.0;
        double lon2 = 9.0;

        // convert to PTV Mercator
        double x1, y1, x2, y2;
        LatLonToSphereMercator(lat1, lon1, out x1, out y1);
        LatLonToSphereMercator(lat2, lon2, out x2, out y2);

        // mercator distance
        double mercDist = Math.Sqrt((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1));
      
        // real distance
        double realDist = mercDist * Math.Cos(lat1 * Math.PI / 180.0);
      
        Console.WriteLine("Distance Between {0}/{1} and {2}/{3}", lat1, lon1, lat2, lon2);
        Console.WriteLine("Mercator: {0}", mercDist);
        Console.WriteLine("Real: {0}", realDist);      
    }

    // Project a geographic coordinate on the (spherical) Mercator map.
    // Using the mean between major an minor axis here ("PTV standard").
    // You could also use the major axis 6378137 ("Google standard").
    public static void LatLonToSphereMercator(double latitude,
                                           double longitude, out double x, out double y)
    {
        x = 6371000.0 * longitude * Math.PI / 180;
        y = 6371000.0 * Math.Log(Math.Tan(Math.PI / 4 + latitude * Math.PI / 360));
    }
}