function E_N_to_Lat_Long($East, $North)
{
$a = 6377563.396;
// Semi-major axis, a
$b = 6356256.91;
//Semi-minor axis, b
$e0 = 400000.0;
//True origin Easting, E0
$n0 = -100000.0;
//True origin Northing, N0
$f0 = 0.9996012717;
//Central Meridan Scale, F0
$PHI0 = 49.0;
// True origin latitude, j0
$LAM0 = -2.0;
// True origin longitude, l0
//Convert angle measures to radians
$RadPHI0 = $PHI0 * (M_PI / 180);
$RadLAM0 = $LAM0 * (M_PI / 180);
//Compute af0, bf0, e squared (e2), n and Et
$af0 = $a * $f0;
$bf0 = $b * $f0;
$e2 = ($af0 * $af0 - $bf0 * $bf0) / ($af0 * $af0);
$n = ($af0 - $bf0) / ($af0 + $bf0);
$Et = $East - $e0;
//Compute initial value for latitude (PHI) in radians
$PHId = InitialLat($North, $n0, $af0, $RadPHI0, $n, $bf0);
$sinPHId2 = pow(sin($PHId), 2);
$cosPHId = pow(cos($PHId), -1);
$tanPHId = tan($PHId);
$tanPHId2 = pow($tanPHId, 2);
$tanPHId4 = pow($tanPHId, 4);
$tanPHId6 = pow($tanPHId, 6);
//Compute nu, rho and eta2 using value for PHId
$nu = $af0 / sqrt(1 - $e2 * $sinPHId2);
$rho = $nu * (1 - $e2) / (1 - $e2 * $sinPHId2);
$eta2 = $nu / $rho - 1;
//Compute Longitude
$X = $cosPHId / $nu;
$XI = $cosPHId / (6 * pow($nu, 3)) * ($nu / $rho + 2 * $tanPHId2);
$XII = $cosPHId / (120 * pow($nu, 5)) * (5 + 28 * $tanPHId2 + 24 * $tanPHId4);
$XIIA = $cosPHId / (5040 * pow($nu, 7)) * (61 + 662 * $tanPHId2 + 1320 * $tanPHId4 + 720 * $tanPHId6);
$VII = $tanPHId / (2 * $rho * $nu);
$VIII = $tanPHId / (24 * $rho * pow($nu, 3)) * (5 + 3 * $tanPHId2 + $eta2 - 9 * $eta2 * $tanPHId2);
$IX = $tanPHId / (720 * $rho * pow($nu, 5)) * (61 + 90 * $tanPHId2 + 45 * $tanPHId4);
$long = 180 / M_PI * ($RadLAM0 + $Et * $X - pow($Et, 3) * $XI + pow($Et, 5) * $XII - pow($Et, 7) * $XIIA);
$lat = 180 / M_PI * ($PHId - pow($Et, 2) * $VII + pow($Et, 4) * $VIII - pow($Et, 6) * $IX);
return array($lat, $long);
}
function NEtoLL($east, $north)
{
// converts NGR easting and nothing to lat, lon.
// input metres, output radians
$nX = (double) $north;
$eX = (double) $east;
$a = 6377563.396;
// OSGB semi-major
$b = 6356256.91;
// OSGB semi-minor
$e0 = 400000;
// OSGB easting of false origin
$n0 = -100000;
// OSGB northing of false origin
$f0 = 0.9996012717;
// OSGB scale factor on central meridian
$e2 = 0.0066705397616;
// OSGB eccentricity squared
$lam0 = -0.03490658503988659;
// OSGB false east
$phi0 = 0.8552113334772214;
// OSGB false north
$af0 = $a * $f0;
$bf0 = $b * $f0;
$n = ($af0 - $bf0) / ($af0 + $bf0);
$Et = $east - $e0;
$phid = InitialLat($north, $n0, $af0, $phi0, $n, $bf0);
$nu = $af0 / sqrt(1 - $e2 * (sin($phid) * sin($phid)));
$rho = $nu * (1 - $e2) / (1 - $e2 * sin($phid) * sin($phid));
$eta2 = $nu / $rho - 1;
$tlat2 = tan($phid) * tan($phid);
$tlat4 = pow(tan($phid), 4);
$tlat6 = pow(tan($phid), 6);
$clatm1 = pow(cos($phid), -1);
$VII = tan($phid) / (2 * $rho * $nu);
$VIII = tan($phid) / (24 * $rho * ($nu * $nu * $nu)) * (5 + 3 * $tlat2 + $eta2 - 9 * $eta2 * $tlat2);
$IX = tan($phid) / (720 * $rho * pow($nu, 5)) * (61 + 90 * $tlat2 + 45 * pow(tan($phid), 4));
$phip = $phid - $Et * $Et * $VII + pow($Et, 4) * $VIII - pow($Et, 6) * $IX;
$X = pow(cos($phid), -1) / $nu;
$XI = $clatm1 / (6 * ($nu * $nu * $nu)) * ($nu / $rho + 2 * $tlat2);
$XII = $clatm1 / (120 * pow($nu, 5)) * (5 + 28 * $tlat2 + 24 * $tlat4);
$XIIA = $clatm1 / (5040 * pow($nu, 7)) * (61 + 662 * $tlat2 + 1320 * $tlat4 + 720 * $tlat6);
$lambdap = $lam0 + $Et * $X - $Et * $Et * $Et * $XI + pow($Et, 5) * $XII - pow($Et, 7) * $XIIA;
$rad2deg = 180.0 / 3.141582;
$geo = array('latitude' => $phip * $rad2deg, 'longitude' => $lambdap * $rad2deg);
return $geo;
}
