public static LatLon ConvertOSToLatLon(double easting, double northing)
{
RefEll airy1830 = new RefEll(6377563.396, 6356256.909);
double OSGB_F0 = 0.9996012717;
double N0 = -100000.0;
double E0 = 400000.0;
double phi0 = Deg2Rad(49.0);
double lambda0 = Deg2Rad(-2.0);
double a = airy1830.maj;
double b = airy1830.min;
double eSquared = airy1830.ecc;
double phi = 0.0;
double lambda = 0.0;
double E = easting;
double N = northing;
double n = (a - b) / (a + b);
double M = 0.0;
double 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))
* Math.Sin(phiPrime - phi0)
* Math.Cos(phiPrime + phi0))
+ ((((15.0 / 8.0) * n * n) + ((15.0 / 8.0) * n * n * n))
* Math.Sin(2.0 * (phiPrime - phi0))
* Math.Cos(2.0 * (phiPrime + phi0)))
- (((35.0 / 24.0) * n * n * n)
* Math.Sin(3.0 * (phiPrime - phi0))
* Math.Cos(3.0 * (phiPrime + phi0))));
phiPrime += (N - N0 - M) / (a * OSGB_F0);
} while ((N - N0 - M) >= 0.001);
var v = a * OSGB_F0 * Math.Pow(1.0 - eSquared * SinSquared(phiPrime), -0.5);
var rho =
a
* OSGB_F0
* (1.0 - eSquared)
* Math.Pow(1.0 - eSquared * SinSquared(phiPrime), -1.5);
var etaSquared = (v / rho) - 1.0;
var VII = Math.Tan(phiPrime) / (2 * rho * v);
var VIII =
(Math.Tan(phiPrime) / (24.0 * rho * Math.Pow(v, 3.0)))
* (5.0
+ (3.0 * TanSquared(phiPrime))
+ etaSquared
- (9.0 * TanSquared(phiPrime) * etaSquared));
var IX =
(Math.Tan(phiPrime) / (720.0 * rho * Math.Pow(v, 5.0)))
* (61.0
+ (90.0 * TanSquared(phiPrime))
+ (45.0 * TanSquared(phiPrime) * TanSquared(phiPrime)));
var X = Sec(phiPrime) / v;
var XI =
(Sec(phiPrime) / (6.0 * v * v * v))
* ((v / rho) + (2 * TanSquared(phiPrime)));
var XII =
(Sec(phiPrime) / (120.0 * Math.Pow(v, 5.0)))
* (5.0
+ (28.0 * TanSquared(phiPrime))
+ (24.0 * TanSquared(phiPrime) * TanSquared(phiPrime)));
var XIIA =
(Sec(phiPrime) / (5040.0 * Math.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 * Math.Pow(E - E0, 2.0))
+ (VIII * Math.Pow(E - E0, 4.0))
- (IX * Math.Pow(E - E0, 6.0));
lambda =
lambda0
+ (X * (E - E0))
- (XI * Math.Pow(E - E0, 3.0))
+ (XII * Math.Pow(E - E0, 5.0))
- (XIIA * Math.Pow(E - E0, 7.0));
lat = Rad2Deg(phi);
lng = Rad2Deg(lambda);
// convert to WGS84
OSGB36ToWGS84();
return new LatLon(lat, lng);
}
private static void OSGB36ToWGS84()
{
var airy1830 = new RefEll(6377563.396, 6356256.909);
var a = airy1830.maj;
var b = airy1830.min;
var eSquared = airy1830.ecc;
var phi = Deg2Rad(lat);
var lambda = Deg2Rad(lng);
var v = a / (Math.Sqrt(1 - eSquared * SinSquared(phi)));
var H = 0; // height
var x = (v + H) * Math.Cos(phi) * Math.Cos(lambda);
var y = (v + H) * Math.Cos(phi) * Math.Sin(lambda);
var z = ((1 - eSquared) * v + H) * Math.Sin(phi);
var tx = 446.448;
var ty = -124.157;
var tz = 542.060;
var s = -0.0000204894;
var rx = Deg2Rad(0.00004172222);
var ry = Deg2Rad(0.00006861111);
var rz = Deg2Rad(0.00023391666);
var xB = tx + (x * (1 + s)) + (-rx * y) + (ry * z);
var yB = ty + (rz * x) + (y * (1 + s)) + (-rx * z);
var zB = tz + (-ry * x) + (rx * y) + (z * (1 + s));
var wgs84 = new RefEll(6378137.000, 6356752.3141);
a = wgs84.maj;
b = wgs84.min;
eSquared = wgs84.ecc;
var lambdaB = Rad2Deg(Math.Atan(yB / xB));
var p = Math.Sqrt((xB * xB) + (yB * yB));
var phiN = Math.Atan(zB / (p * (1 - eSquared)));
for (var i = 1; i < 10; i++)
{
v = a / (Math.Sqrt(1 - eSquared * SinSquared(phiN)));
double phiN1 = Math.Atan((zB + (eSquared * v * Math.Sin(phiN))) / p);
phiN = phiN1;
}
var phiB = Rad2Deg(phiN);
lat = phiB;
lng = lambdaB;
}
