Use the following method to determine the "great circle" distance, in miles, between two geological coordinates. Latitude is positive north of the equator. Longitude is negative west of Greenwich:

public static double DistanceBetweenLocations(double Lat1Degrees, 
                                              double Lon1Degrees, 
                                              double Lat2Degrees, 
                                              double Lon2Degrees)
{
  double Angle = AngleBetweenLocations(Lat1Degrees, Lon1Degrees,
                                       Lat2Degrees, Lon2Degrees);
    
  double Circumference = 24830.0; // miles at equator
    
  return Circumference * Angle / (2.0 * Math.PI);
}

public static double AngleBetweenLocations(double Lat1Degrees, 
                                           double Lon1Degrees, 
                                           double Lat2Degrees, 
                                           double Lon2Degrees)
{
  double Lat1Radians = Radians(Lat1Degrees);
  double Lon1Radians = Radians(Lon1Degrees);
  double Lat2Radians = Radians(Lat2Degrees);
  double Lon2Radians = Radians(Lon2Degrees);
    
  double a = Lon1Radians - Lon2Radians;
    
  if (a < 0.0)
  {
    a = -a;
  }
    
  if (a > Math.PI)
  {
    a = 2.0 * Math.PI - a;
  }
    
  return Math.Acos(
        Math.Sin(Lat2Radians) * Math.Sin(Lat1Radians) + 
        Math.Cos(Lat2Radians) * Math.Cos(Lat1Radians) * Math.Cos(a)
        );
}

public static double Radians(double degrees)
{
  return degrees * Math.PI / 180.0;
}