calculate Hubeny's distance from Geolocation

gistfile1.js

// http://yamadarake.web.fc2.com/trdi/2009/report000001.html
function calcHubenyDistance(lat1, lon1, lat2, lon2){
	var a = 6378137.000;
	var b = 6356752.314245;
	var e2= 0.00669437999019758;
	
	var y1	= lat1 * Math.PI / 180;
	var x1 = lon1 * Math.PI / 180;
	var y2 = lat2 * Math.PI / 180;
	var x2 = lon2 * Math.PI / 180;

	var y_ave = (y1 + y2) / 2;
	var y_diff = y1 - y2;
	var x_diff = x1 - x2;
	
	var w = Math.sqrt(1 - e2 * Math.pow(Math.sin(y_ave), 2));
	var n = a / w;
	var m = a * (1 - e2) / Math.pow(w, 3);

	return Math.sqrt(Math.pow(y_diff * m, 2) + Math.pow(x_diff * n *Math.cos(y_ave), 2));
}