I have a program that needs to compute the distance between two locations using their GPS coordinates. I had attempted to use the Halverson formula, but my calculations are off by many miles, almost 30%. My code (in C#) is as follows:

private double Calc(double Lat1, double Long1, double Lat2, double Long2)

{

/*

The Haversine formula according to Dr. Math.

http://mathforum.org/library/drmath/view/51879.html
dlon = lon2 - lon1

dlat = lat2 - lat1

a = (sin(dlat/2))^2 + cos(lat1) * cos(lat2) * (sin(dlon/2))^2

c = 2 * atan2(sqrt(a), sqrt(1-a))

d = R * c

Where

* dlon is the change in longitude

* dlat is the change in latitude

* c is the great circle distance in Radians.

* R is the radius of a spherical Earth.

* The locations of the two points in spherical coordinates (longitude and latitude) are lon1,lat1 and lon2, lat2.

*/

double dDistance = Double.MinValue;

double dLat1InRad = Lat1 * (Math.PI / 180.0);

double dLong1InRad = Long1 * (Math.PI / 180.0);

double dLat2InRad = Lat2 * (Math.PI / 180.0);

double dLong2InRad = Long2 * (Math.PI / 180.0);

double dLongitude = dLong2InRad - dLong1InRad;

double dLatitude = dLat2InRad - dLat1InRad;

// Intermediate result a.

double a = Math.Pow(Math.Sin(dLatitud

e / 2.0), 2.0) +

Math.Cos(dLat1InRad) * Math.Cos(dLat2InRad) *

Math.Pow(Math.Sin(dLongitude / 2.0), 2.0);

// Intermediate result c (great circle distance in Radians).

double c = 2.0 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1.0 - a));

// Distance.

// const Double kEarthRadiusMiles = 3956.0;

const Double kEarthRadiusKms = 6376.5;

dDistance = kEarthRadiusKms * c;

return dDistance;

}

//----------------- computes distance using direction ---------------------

public double Calc(string NS1, double Lat1, double Lat1Min, string EW1, double Long1, double Long1Min,

string NS2, double Lat2, double Lat2Min, string EW2, double Long2, double Long2Min)

{

// usage: ComputeDistance.Calc(" N 43 35.500 W 80 27.800 N 43 35.925 W 80 28.318");

double NS1Sign = NS1.ToUpper() == "N" ? 1.0 : -1.0;

double EW1Sign = NS1.ToUpper() == "E" ? 1.0 : -1.0;

double NS2Sign = NS2.ToUpper() == "N" ? 1.0 : -1.0;

double EW2Sign = EW2.ToUpper() == "E" ? 1.0 : -1.0;

return (Calc( (Lat1 + (Lat1Min / 60)) * NS1Sign,

(Long1 + (Long1Min / 60)) * EW1Sign,

(Lat2 + (Lat2Min / 60)) * NS2Sign,

(Long2 + (Long2Min / 60)) * EW2Sign

));

}

Can someone please tell me where my error is? (I am not a math major, nor anywhere close to one).

Thanks...