Function for calculating Earth radius at a specific geodetic latitude.

EarthRadius.js

function EarthRadiusInMeters(latitudeRadians)
{
    // latitudeRadians is geodetic, i.e. that reported by GPS.
    // http://en.wikipedia.org/wiki/Earth_radius
    var a = 6378137.0;  // equatorial radius in meters
    var b = 6356752.3;  // polar radius in meters
    var cos = Math.cos(latitudeRadians);
    var sin = Math.sin(latitudeRadians);
    var t1 = a * a * cos;
    var t2 = b * b * sin;
    var t3 = a * cos;
    var t4 = b * sin;
    return Math.sqrt((t1*t1 + t2*t2) / (t3*t3 + t4*t4));
}