I'm calculating the distance between two GeoCoordinates. I'm testing my app against 3-4 other apps. When I'm calculating distance, I tend to get an average of 3.3 miles for my calculation whereas other apps are getting 3.5 miles. It's a big difference for the calculation I'm trying to perform. Are there any good class libraries out there for calculating distance? I'm calculating it like this in C#:

```
public static double Calculate(double sLatitude,double sLongitude, double eLatitude,
double eLongitude)
{
var radiansOverDegrees = (Math.PI / 180.0);
var sLatitudeRadians = sLatitude * radiansOverDegrees;
var sLongitudeRadians = sLongitude * radiansOverDegrees;
var eLatitudeRadians = eLatitude * radiansOverDegrees;
var eLongitudeRadians = eLongitude * radiansOverDegrees;
var dLongitude = eLongitudeRadians - sLongitudeRadians;
var dLatitude = eLatitudeRadians - sLatitudeRadians;
var result1 = Math.Pow(Math.Sin(dLatitude / 2.0), 2.0) +
Math.Cos(sLatitudeRadians) * Math.Cos(eLatitudeRadians) *
Math.Pow(Math.Sin(dLongitude / 2.0), 2.0);
// Using 3956 as the number of miles around the earth
var result2 = 3956.0 * 2.0 *
Math.Atan2(Math.Sqrt(result1), Math.Sqrt(1.0 - result1));
return result2;
}
```

What could I be doing wrong? Should I calculate it in km first and then convert to miles?

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