public static function tsfnz($eccent, $phi, $sinphi)
    {
        $con = $eccent * $sinphi;
        $com = 0.5 * $eccent;
        $con = pow(((1.0 - $con) / (1.0 + $con)), $com);

        return (tan(0.5 * (M_PI_2 - $phi) ) / $con);
    }