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For the computation of the distance between 2 GPS points, it would be more precise to consider the WGS84 model to compute the local earth radius instead of considering the earth radius as a constant.

Here is a C coding of the function that computes the local earth radius as a function of the latitude (in radian).

One can take (local_earth_radius(lat1)+local_earth_radius(lat2))/2 as the earth radius in the method computeDistance().

double local_earth_radius(double lat_rad)

{

// WGS84 data

double a = 6378137.0;

double f = 1./298.257222101;

double e = sqrt(2.*f-f*f);

double R1, R2;

double dum;

dum = 1. -e*e*sin(lat_rad)*sin(lat_rad);

R1 = a * (1. - e*e) / ( dum*sqrt(dum) );

R2 = a / sqrt(dum);

return( sqrt(R1*R2) );

}