Calculate the distance between two points on the globe using Haversine and Vincenty formulas.

Haversine.cs

#region Haversine
/// <summary>
/// Gets the distance between two points in meters using the Haversine Formula.
/// </summary>
/// <param name="startLong">The start longitude.</param>
/// <param name="startLat">The start latitude.</param>
/// <param name="endLat">The end latitude.</param>
/// <param name="endLong">The end longitude.</param>
/// <returns>Distance between the two points in meters</returns>
public static double GetDistanceHaversine(double startLong, double startLat, double endLat, double endLong)
{
	double lon1 = startLong / 180 * Math.PI;
	double lon2 = endLong / 180 * Math.PI;
	double lat1 = startLat / 180 * Math.PI;
	double lat2 = endLat / 180 * Math.PI;
	return 2 * Math.Asin(Math.Sqrt(Math.Pow((Math.Sin((lat1 - lat2) / 2)), 2) +
	 Math.Cos(lat1) * Math.Cos(lat2) * Math.Pow(Math.Sin((lon1 - lon2) / 2), 2))) * EarthCircumference;
}
#endregion

Vincenty.cs

#region Vincenty
private const double D2R = 0.0174532925; //Degrees to radians
private const double semiMajor = 6378137; //Earth radius in meters (a)  (WGS84)
private const double semiMinor = 6356752.314245; //Earth radius in meters (b)  (WGS84)
private const double flattening = 1 / 298.257223563; //Flattening (WGS84): 1 / 298.257223563; 

/// <summary>
/// Vincenty's formulae is used in geodesy to calculate the distance
/// between two points on the surface of an spheroid.
/// </summary>
/// <param name="lat1">The start latitude.</param>
/// <param name="lon1">The start longitude.</param>
/// <param name="lat2">The end latitude.</param>
/// <param name="lon2">The end longitude.</param>
/// <returns>Distance in meters between the two points</returns>
/// <remarks>
/// Developed by Thaddeus Vincenty in 1975. They are based on the 
/// assumption that the figure of the Earth is an oblate spheroid,
/// and hence are more accurate than methods such as great-circle 
/// distance which assume a spherical Earth.
/// It is considered to be accurate to within 0.5 mm on the Earth Ellipsoid.
/// </remarks>
public static double GetDistanceVincenty(double lat1, double lon1, double lat2, double lon2)
{
	var a = semiMajor;
	var b = semiMinor;
	var f = flattening;
	var L = (lon2 - lon1) * D2R;
	var U1 = Math.Atan((1 - f) * Math.Tan(lat1 * D2R));
	var U2 = Math.Atan((1 - f) * Math.Tan(lat2 * D2R));
	var sinU1 = Math.Sin(U1);
	var cosU1 = Math.Cos(U1);
	var sinU2 = Math.Sin(U2);
	var cosU2 = Math.Cos(U2);

	var lambda = L;
	double lambdaP;
	double cosSigma, cosSqAlpha, sinSigma, cos2SigmaM, sigma, sinLambda, cosLambda;

	int iterLimit = 100;
	do
	{
		sinLambda = Math.Sin(lambda);
		cosLambda = Math.Cos(lambda);
		sinSigma = Math.Sqrt((cosU2 * sinLambda) * (cosU2 * sinLambda) +
			(cosU1 * sinU2 - sinU1 * cosU2 * cosLambda) * (cosU1 * sinU2 - sinU1 * cosU2 * cosLambda));
		if (sinSigma == 0)
			return 0;  // co-incident points

		cosSigma = sinU1 * sinU2 + cosU1 * cosU2 * cosLambda;
		sigma = Math.Atan2(sinSigma, cosSigma);
		double sinAlpha = cosU1 * cosU2 * sinLambda / sinSigma;
		cosSqAlpha = 1 - sinAlpha * sinAlpha;
		cos2SigmaM = cosSigma - 2 * sinU1 * sinU2 / cosSqAlpha;
		if (double.IsNaN(cos2SigmaM))
			cos2SigmaM = 0;  // equatorial line: cosSqAlpha=0 (§6)
		double C = f / 16 * cosSqAlpha * (4 + f * (4 - 3 * cosSqAlpha));
		lambdaP = lambda;
		lambda = L + (1 - C) * f * sinAlpha *
			(sigma + C * sinSigma * (cos2SigmaM + C * cosSigma * (-1 + 2 * cos2SigmaM * cos2SigmaM)));
	} while (Math.Abs(lambda - lambdaP) > 1e-12 && --iterLimit > 0);

	if (iterLimit == 0) return double.NaN;  // formula failed to converge

	var uSq = cosSqAlpha * (a * a - b * b) / (b * b);
	var A = 1 + uSq / 16384 * (4096 + uSq * (-768 + uSq * (320 - 175 * uSq)));
	var B = uSq / 1024 * (256 + uSq * (-128 + uSq * (74 - 47 * uSq)));
	var deltaSigma = B * sinSigma * (cos2SigmaM + B / 4 * (cosSigma * (-1 + 2 * cos2SigmaM * cos2SigmaM) -
		B / 6 * cos2SigmaM * (-3 + 4 * sinSigma * sinSigma) * (-3 + 4 * cos2SigmaM * cos2SigmaM)));
	var s = b * A * (sigma - deltaSigma);

	s = Math.Round(s, 3); // round to 1mm precision
	return s;
}
#endregion