Here is a function in PHP that returns the distance between two points on earth based on the WGS84 datum. This is nothing new. It is called the Vincenty's folmula and the theory can be found at
http://www.ngs.noaa.gov/PUBS_LIB/inverse.pdf
This is maybe not what you're looking for but it can be easily translated in any other language and is accurate to less than a meter. You just need to send the latitude and longitude in decimal degrees.
function distInvVincenty($lat1, $lon1, $lat2, $lon2) {
/* WGS84 stuff */
$a = 6378137;
$b = 6356752.3142;
$f = 1/298.257223563;
/* end of WGS84 stuff */
$L = deg2rad($lon2-$lon1);
$U1 = atan((1-$f) * tan(deg2rad($lat1)));
$U2 = atan((1-$f) * tan(deg2rad($lat2)));
$sinU1 = sin($U1);
$cosU1 = cos($U1);
$sinU2 = sin($U2);
$cosU2 = cos($U2);
$lambda = $L;
$lambdaP = 2*pi();
$iterLimit = 20;
while ((abs($lambda-$lambdaP) > pow(10, -12)) && ($iterLimit-- > 0)) {
$sinLambda = sin($lambda);
$cosLambda = cos($lambda);
$sinSigma = sqrt(($cosU2*$sinLambda) * ($cosU2*$sinLambda) + ($cosU1*$sinU2-$sinU1*$cosU2*$cosLambda) * ($cosU1*$sinU2-$sinU1*$cosU2*$cosLambda));
if ($sinSigma==0) {return 0;}
$cosSigma = $sinU1*$sinU2 + $cosU1*$cosU2*$cosLambda;
$sigma = atan2($sinSigma, $cosSigma);
$sinAlpha = $cosU1 * $cosU2 * $sinLambda / $sinSigma;
$cosSqAlpha = 1 - $sinAlpha*$sinAlpha;
$cos2SigmaM = $cosSigma - 2*$sinU1*$sinU2/$cosSqAlpha;
if (is_nan($cos2SigmaM)) {$cos2SigmaM = 0;}
$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*$cos2SigmaM)));
}
if ($iterLimit==0) {return NaN;} // formula failed to converge
$uSq = $cosSqAlpha * ($a*$a - $b*$b) / ($b*$b);
$A = 1 + $uSq/16384*(4096+$uSq*(-768+$uSq*(320-175*$uSq)));
$B = $uSq/1024 * (256+$uSq*(-128+$uSq*(74-47*$uSq)));
$deltaSigma = $B*$sinSigma*($cos2SigmaM+$B/4*($cosSigma*(-1+2*$cos2SigmaM*$cos2SigmaM)- $B/6*$cos2SigmaM*(-3+4*$sinSigma*$sinSigma)*(-3+4*$cos2SigmaM*$cos2SigmaM)));
$s = $b*$A*($sigma-$deltaSigma);
return "Distance: ".$s;
}