/**
* Convert this grid reference into a latitude and longitude
*
* @return
*/
function toLatLng()
{
$airy1830 = new RefEll(6377563.396, 6356256.909);
$OSGB_F0 = 0.9996012717;
$N0 = -100000.0;
$E0 = 400000.0;
$phi0 = deg2rad(49.0);
$lambda0 = deg2rad(-2.0);
$a = $airy1830->maj;
$b = $airy1830->min;
$eSquared = $airy1830->ecc;
$phi = 0.0;
$lambda = 0.0;
$E = $this->easting;
$N = $this->northing;
$n = ($a - $b) / ($a + $b);
$M = 0.0;
$phiPrime = ($N - $N0) / ($a * $OSGB_F0) + $phi0;
do {
$M = $b * $OSGB_F0 * ((1 + $n + 5.0 / 4.0 * $n * $n + 5.0 / 4.0 * $n * $n * $n) * ($phiPrime - $phi0) - (3 * $n + 3 * $n * $n + 21.0 / 8.0 * $n * $n * $n) * sin($phiPrime - $phi0) * cos($phiPrime + $phi0) + (15.0 / 8.0 * $n * $n + 15.0 / 8.0 * $n * $n * $n) * sin(2.0 * ($phiPrime - $phi0)) * cos(2.0 * ($phiPrime + $phi0)) - 35.0 / 24.0 * $n * $n * $n * sin(3.0 * ($phiPrime - $phi0)) * cos(3.0 * ($phiPrime + $phi0)));
$phiPrime += ($N - $N0 - $M) / ($a * $OSGB_F0);
} while ($N - $N0 - $M >= 0.001);
$v = $a * $OSGB_F0 * pow(1.0 - $eSquared * sinSquared($phiPrime), -0.5);
$rho = $a * $OSGB_F0 * (1.0 - $eSquared) * pow(1.0 - $eSquared * sinSquared($phiPrime), -1.5);
$etaSquared = $v / $rho - 1.0;
$VII = tan($phiPrime) / (2 * $rho * $v);
$VIII = tan($phiPrime) / (24.0 * $rho * pow($v, 3.0)) * (5.0 + 3.0 * tanSquared($phiPrime) + $etaSquared - 9.0 * tanSquared($phiPrime) * $etaSquared);
$IX = tan($phiPrime) / (720.0 * $rho * pow($v, 5.0)) * (61.0 + 90.0 * tanSquared($phiPrime) + 45.0 * tanSquared($phiPrime) * tanSquared($phiPrime));
$X = sec($phiPrime) / $v;
$XI = sec($phiPrime) / (6.0 * $v * $v * $v) * ($v / $rho + 2 * tanSquared($phiPrime));
$XII = sec($phiPrime) / (120.0 * pow($v, 5.0)) * (5.0 + 28.0 * tanSquared($phiPrime) + 24.0 * tanSquared($phiPrime) * tanSquared($phiPrime));
$XIIA = sec($phiPrime) / (5040.0 * pow($v, 7.0)) * (61.0 + 662.0 * tanSquared($phiPrime) + 1320.0 * tanSquared($phiPrime) * tanSquared($phiPrime) + 720.0 * tanSquared($phiPrime) * tanSquared($phiPrime) * tanSquared($phiPrime));
$phi = $phiPrime - $VII * pow($E - $E0, 2.0) + $VIII * pow($E - $E0, 4.0) - $IX * pow($E - $E0, 6.0);
$lambda = $lambda0 + $X * ($E - $E0) - $XI * pow($E - $E0, 3.0) + $XII * pow($E - $E0, 5.0) - $XIIA * pow($E - $E0, 7.0);
return new LatLng(rad2deg($phi), rad2deg($lambda));
}
/**
* Convert this LatLng object from OSGB36 datum to WGS84 datum.
*/
function OSGB36ToWGS84()
{
$airy1830 = new RefEll(6377563.396, 6356256.909);
$a = $airy1830->maj;
$b = $airy1830->min;
$eSquared = $airy1830->ecc;
$phi = deg2rad($this->lat);
$lambda = deg2rad($this->lng);
$v = $a / sqrt(1 - $eSquared * coordHelpers::sinSquared($phi));
$H = 0;
// height
$x = ($v + $H) * cos($phi) * cos($lambda);
$y = ($v + $H) * cos($phi) * sin($lambda);
$z = ((1 - $eSquared) * $v + $H) * sin($phi);
$tx = 446.448;
$ty = -124.157;
$tz = 542.0599999999999;
$s = -2.04894E-5;
$rx = deg2rad(4.172222E-5);
$ry = deg2rad(6.861111000000001E-5);
$rz = deg2rad(0.00023391666);
$xB = $tx + $x * (1 + $s) + -$rx * $y + $ry * $z;
$yB = $ty + $rz * $x + $y * (1 + $s) + -$rx * $z;
$zB = $tz + -$ry * $x + $rx * $y + $z * (1 + $s);
$wgs84 = new RefEll(6378137.0, 6356752.3141);
$a = $wgs84->maj;
$b = $wgs84->min;
$eSquared = $wgs84->ecc;
$lambdaB = rad2deg(atan($yB / $xB));
$p = sqrt($xB * $xB + $yB * $yB);
$phiN = atan($zB / ($p * (1 - $eSquared)));
for ($i = 1; $i < 10; $i++) {
$v = $a / sqrt(1 - $eSquared * sinSquared($phiN));
$phiN1 = atan(($zB + $eSquared * $v * sin($phiN)) / $p);
$phiN = $phiN1;
}
$phiB = rad2deg($phiN);
$this->lat = $phiB;
$this->lng = $lambdaB;
}
