function OGBLatLng(lat, lon) {
    this.lat = lat;
    this.lon = lon;
}

function WGS84LatLng(lat, lon) {
    this.lat = lat;
    this.lon = lon;
}

function OGBNorthEast(east, north) {
    this.north = north;
    this.east = east;
}

function OGBRect(bottomLeft, topRight) {
    this.bl = bottomLeft;
    this.tr = topRight;
}



//convert WGS84 Latitude and Longitude to Ordnance Survey 1936 Latitude Longitude

function WGS84ToOGB(WGlat, WGlon, height) {
    var deg2rad = Math.PI / 180;
    var rad2deg = 180.0 / Math.PI;

    //first off convert to radians
    var radWGlat = WGlat * deg2rad;
    var radWGlon = WGlon * deg2rad;
    //these are the values for WGS84(GRS80) to OSGB36(Airy)
    var a = 6378137; // WGS84_AXIS
    var e = 0.00669438037928458; // WGS84_ECCENTRIC
    var h = height; // height above datum (from $GPGGA sentence)
    var a2 = 6377563.396; // OSGB_AXIS
    var e2 = 0.0066705397616; // OSGB_ECCENTRIC 
    var xp = -446.448;
    var yp = 125.157;
    var zp = -542.06;
    var xr = -0.1502;
    var yr = -0.247;
    var zr = -0.8421;
    var s = 20.4894;

    // convert to cartesian; lat, lon are in radians
    var sf = s * 0.000001;
    var v = a / (Math.sqrt(1 - (e * (Math.sin(radWGlat) * Math.sin(radWGlat)))));
    var x = (v + h) * Math.cos(radWGlat) * Math.cos(radWGlon);
    var y = (v + h) * Math.cos(radWGlat) * Math.sin(radWGlon);
    var z = ((1 - e) * v + h) * Math.sin(radWGlat);

    // transform cartesian
    var xrot = (xr / 3600) * deg2rad;
    var yrot = (yr / 3600) * deg2rad;
    var zrot = (zr / 3600) * deg2rad;
    var hx = x + (x * sf) - (y * zrot) + (z * yrot) + xp;
    var hy = (x * zrot) + y + (y * sf) - (z * xrot) + yp;
    var hz = (-1 * x * yrot) + (y * xrot) + z + (z * sf) + zp;

    // Convert back to lat, lon
    var newLon = Math.atan(hy / hx);
    var p = Math.sqrt((hx * hx) + (hy * hy));
    var newLat = Math.atan(hz / (p * (1 - e2)));
    v = a2 / (Math.sqrt(1 - e2 * (Math.sin(newLat) * Math.sin(newLat))));
    var errvalue = 1.0;
    var lat0 = 0;
    while (errvalue > 0.001) {
        lat0 = Math.atan((hz + e2 * v * Math.sin(newLat)) / p);
        errvalue = Math.abs(lat0 - newLat);
        newLat = lat0;
    }

    //convert back to degrees
    newLat = newLat * rad2deg;
    newLon = newLon * rad2deg;

    return new OGBLatLng(newLat, newLon);

}


//convert Ordnance Survey 1936 Latitude Longitude to WGS84 Latitude and Longitude

function OGBToWGS84(OGlat, OGlon, height) {
    var deg2rad = Math.PI / 180;
    var rad2deg = 180.0 / Math.PI;

    //first off convert to radians
    var radOGlat = OGlat * deg2rad;
    var radOGlon = OGlon * deg2rad;
    //these are the values for WGS84(GRS80) to OSGB36(Airy)

    var a2 = 6378137; // WGS84_AXIS
    var e2 = 0.00669438037928458; // WGS84_ECCENTRIC

    var h = height; // height above datum (from $GPGGA sentence)

    var a = 6377563.396; // OSGB_AXIS
    var e = 0.0066705397616; // OSGB_ECCENTRIC 

    var xp = 446.448;
    var yp = -125.157;
    var zp = 542.06;
    var xr = 0.1502;
    var yr = 0.247;
    var zr = 0.8421;

    var s = -20.4894;

    // convert to cartesian; lat, lon are in radians
    var sf = s * 0.000001;
    var v = a / (Math.sqrt(1 - (e * (Math.sin(radOGlat) * Math.sin(radOGlat)))));
    var x = (v + h) * Math.cos(radOGlat) * Math.cos(radOGlon);
    var y = (v + h) * Math.cos(radOGlat) * Math.sin(radOGlon);
    var z = ((1 - e) * v + h) * Math.sin(radOGlat);

    // transform cartesian
    var xrot = (xr / 3600) * deg2rad;
    var yrot = (yr / 3600) * deg2rad;
    var zrot = (zr / 3600) * deg2rad;
    var hx = x + (x * sf) - (y * zrot) + (z * yrot) + xp;
    var hy = (x * zrot) + y + (y * sf) - (z * xrot) + yp;
    var hz = (-1 * x * yrot) + (y * xrot) + z + (z * sf) + zp;

    // Convert back to lat, lon
    var newLon = Math.atan(hy / hx);
    var p = Math.sqrt((hx * hx) + (hy * hy));
    var newLat = Math.atan(hz / (p * (1 - e2)));
    v = a2 / (Math.sqrt(1 - e2 * (Math.sin(newLat) * Math.sin(newLat))));
    var errvalue = 1.0;
    var lat0 = 0;
    while (errvalue > 0.001) {
        lat0 = Math.atan((hz + e2 * v * Math.sin(newLat)) / p);
        errvalue = Math.abs(lat0 - newLat);
        newLat = lat0;
    }

    //convert back to degrees
    newLat = newLat * rad2deg;
    newLon = newLon * rad2deg;

    return new WGS84LatLng(newLat, newLon);

}


//converts lat and lon (OSGB36) to OS northings and eastings
function LLtoNE(lat, lon) {
    var deg2rad = Math.PI / 180;
    var rad2deg = 180.0 / Math.PI;

    var phi = lat * deg2rad; // convert latitude to radians
    var lam = lon * deg2rad; // convert longitude to radians
    var a = 6377563.396; // OSGB semi-major axis
    var b = 6356256.91; // OSGB semi-minor axis
    var e0 = 400000; // easting of false origin
    var n0 = -100000; // northing of false origin
    var f0 = 0.9996012717; // OSGB scale factor on central meridian
    var e2 = 0.0066705397616; // OSGB eccentricity squared
    var lam0 = -0.034906585039886591; // OSGB false east
    var phi0 = 0.85521133347722145; // OSGB false north
    var af0 = a * f0;
    var bf0 = b * f0;

    // easting
    var slat2 = Math.sin(phi) * Math.sin(phi);
    var nu = af0 / (Math.sqrt(1 - (e2 * (slat2))));
    var rho = (nu * (1 - e2)) / (1 - (e2 * slat2));
    var eta2 = (nu / rho) - 1;
    var p = lam - lam0;
    var IV = nu * Math.cos(phi);
    var clat3 = Math.pow(Math.cos(phi), 3);
    var tlat2 = Math.tan(phi) * Math.tan(phi);
    var V = (nu / 6) * clat3 * ((nu / rho) - tlat2);
    var clat5 = Math.pow(Math.cos(phi), 5);
    var tlat4 = Math.pow(Math.tan(phi), 4);
    var VI = (nu / 120) * clat5 * ((5 - (18 * tlat2)) + tlat4 + (14 * eta2) - (58 * tlat2 * eta2));
    var east = e0 + (p * IV) + (Math.pow(p, 3) * V) + (Math.pow(p, 5) * VI);

    // northing
    var n = (af0 - bf0) / (af0 + bf0);
    var M = Marc(bf0, n, phi0, phi);
    var I = M + (n0);
    var II = (nu / 2) * Math.sin(phi) * Math.cos(phi);
    var III = ((nu / 24) * Math.sin(phi) * Math.pow(Math.cos(phi), 3)) * (5 - Math.pow(Math.tan(phi), 2) + (9 * eta2));
    var IIIA = ((nu / 720) * Math.sin(phi) * clat5) * (61 - (58 * tlat2) + tlat4);
    var north = I + ((p * p) * II) + (Math.pow(p, 4) * III) + (Math.pow(p, 6) * IIIA);

    // make whole number values
    east = Math.round(east); // round to whole number of meters
    north = Math.round(north);

    return new OGBNorthEast(east, north);
}


//convert northing and easting to letter and number grid system
function NE2NGR(east, north) {
    east = Math.round(east);
    north = Math.round(north);
    var eX = east / 500000;
    var nX = north / 500000;
    var tmp = Math.floor(eX) - 5.0 * Math.floor(nX) + 17.0;
    nX = 5 * (nX - Math.floor(nX));
    eX = 20 - 5.0 * Math.floor(nX) + Math.floor(5.0 * (eX - Math.floor(eX)));
    if (eX > 7.5) eX = eX + 1; // I is not used
    if (tmp > 7.5) tmp = tmp + 1; // I is not used

    var eing = east - (Math.floor(east / 100000) * 100000);
    var ning = north - (Math.floor(north / 100000) * 100000);
    var estr = eing.toString();
    var nstr = ning.toString();
    while (estr.length < 5)
        estr = "0" + estr;
    while (nstr.length < 5)
        nstr = "0" + nstr;

    var ngr = String.fromCharCode(tmp + 65) +
          String.fromCharCode(eX + 65) +
          " " + estr + " " + nstr;

    return ngr;
}

//helper
function Marc(bf0, n, phi0, phi) {
    return bf0 * (((1 + n + ((5 / 4) * (n * n)) + ((5 / 4) * (n * n * n))) * (phi - phi0))
- (((3 * n) + (3 * (n * n)) + ((21 / 8) * (n * n * n))) * (Math.sin(phi - phi0)) * (Math.cos(phi + phi0)))
+ ((((15 / 8) * (n * n)) + ((15 / 8) * (n * n * n))) * (Math.sin(2 * (phi - phi0))) * (Math.cos(2 * (phi + phi0))))
- (((35 / 24) * (n * n * n)) * (Math.sin(3 * (phi - phi0))) * (Math.cos(3 * (phi + phi0)))));
}

function NEtoLL(east, north) {

    //metres in, degrees out

    var K0 = 0.9996012717; // grid scale factor on central meridean
    var OriginLat = 49.0;
    var OriginLong = -2.0;
    var OriginX = 400000; // 400 kM
    var OriginY = -100000; // 100 kM
    var a = 6377563.396; // Airy Spheroid
    var b = 6356256.910;

    var e2;
    var ex;
    var n1;
    var n2;
    var n3;
    var OriginNorthings;


    // compute interim values
    a = a * K0;
    b = b * K0;

    n1 = (a - b) / (a + b);
    n2 = n1 * n1;
    n3 = n2 * n1;

    lat = OriginLat * Math.PI / 180.0; // to radians                                                    


    e2 = (a * a - b * b) / (a * a);  // first eccentricity
    ex = (a * a - b * b) / (b * b);  // second eccentricity


    OriginNorthings = b * lat + b * (n1 * (1.0 + 5.0 * n1 * (1.0 + n1) / 4.0) * lat
					- 3.0 * n1 * (1.0 + n1 * (1.0 + 7.0 * n1 / 8.0)) * Math.sin(lat) * Math.cos(lat)
					+ (15.0 * n1 * (n1 + n2) / 8.0) * Math.sin(2.0 * lat) * Math.cos(2.0 * lat)
					- (35.0 * n3 / 24.0) * Math.sin(3.0 * lat) * Math.cos(3.0 * lat));

    var OriginLat = 49.0;
    var OriginLong = -2.0;
    var OriginX = 400000; // 400 kM
    var OriginY = -100000; // 100 kM


    var lat;    // what we calculate
    var lon;

    var northing = north - OriginY;
    var easting = east - OriginX;

    var nu, phid, phid2, t2, t, q2, c, s, nphid, dnphid; // temps
    var nu2, nudivrho, invnurho, rho, eta2;


    /* Evaluate M term: latitude of the northing on the centre meridian */

    northing += OriginNorthings;

    phid = northing / (b * (1.0 + n1 + 5.0 * (n2 + n3) / 4.0)) - 1.0;
    phid2 = phid + 1.0;

    while (Math.abs(phid2 - phid) > 1E-6) {
        phid = phid2;
        nphid = b * phid + b * (n1 * (1.0 + 5.0 * n1 * (1.0 + n1) / 4.0) * phid
           - 3.0 * n1 * (1.0 + n1 * (1.0 + 7.0 * n1 / 8.0)) * Math.sin(phid) * Math.cos(phid)
           + (15.0 * n1 * (n1 + n2) / 8.0) * Math.sin(2.0 * phid) * Math.cos(2.0 * phid)
           - (35.0 * n3 / 24.0) * Math.sin(3.0 * phid) * Math.cos(3.0 * phid));

        dnphid = b * ((1.0 + n1 + 5.0 * (n2 + n3) / 4.0) - 3.0 * (n1 + n2 + 7.0 * n3 / 8.0) * Math.cos(2.0 * phid)
            + (15.0 * (n2 + n3) / 4.0) * Math.cos(4 * phid) - (35.0 * n3 / 8.0) * Math.cos(6.0 * phid));

        phid2 = phid - (nphid - northing) / dnphid;
    }

    c = Math.cos(phid);
    s = Math.sin(phid);
    t = Math.tan(phid);
    t2 = t * t;
    q2 = easting * easting;


    nu2 = (a * a) / (1.0 - e2 * s * s);
    nu = Math.sqrt(nu2);

    nudivrho = a * a * c * c / (b * b) - c * c + 1.0;

    eta2 = nudivrho - 1;

    rho = nu / nudivrho;

    invnurho = ((1.0 - e2 * s * s) * (1.0 - e2 * s * s)) / (a * a * (1.0 - e2));

    lat = phid - t * q2 * invnurho / 2.0 + (q2 * q2 * (t / (24 * rho * nu2 * nu) * (5 + (3 * t2) + eta2 - (9 * t2 * eta2))));

    lon = (easting / (c * nu))
        - (easting * q2 * ((nudivrho + 2.0 * t2) / (6.0 * nu2)) / (c * nu))
		+ (q2 * q2 * easting * (5 + (28 * t2) + (24 * t2 * t2)) / (120 * nu2 * nu2 * nu * c));

    return new OGBLatLng(lat * 180.0 / Math.PI, (lon * 180.0 / Math.PI) + OriginLong);

}

//return rect in OSGB N/E coords that encloses the given wgs84 rect
function enclosingOsgbRect(WGleft, WGbottom, WGtop, WGright) {

    var blOGB = WGS84ToOGB(WGbottom, WGleft, 0);
    var trOGB = WGS84ToOGB(WGtop, WGright, 0);
    var brOGB = WGS84ToOGB(WGbottom, WGright, 0);
    var tlOGB = WGS84ToOGB(WGtop, WGleft, 0);

    var blEN = LLtoNE(blOGB.lat, blOGB.lon);
    var trEN = LLtoNE(trOGB.lat, trOGB.lon);
    var brEN = LLtoNE(brOGB.lat, brOGB.lon);
    var tlEN = LLtoNE(tlOGB.lat, tlOGB.lon);

    var e = Math.min(blEN.east, tlEN.east);
    var w = Math.max(brEN.east, trEN.east);
    var s = Math.min(blEN.north, brEN.north);
    var n = Math.max(trEN.north, tlEN.north);

    return new OGBRect(new OGBNorthEast(e, s), new OGBNorthEast(w, n));
}

function OsgbConvergence(lat, lon) {

    var K0 = 0.9996012717; // grid scale factor on central meridean
    var a = 6377563.396; // Airy Spheroid
    var b = 6356256.910;
    var oDeg = -2.0; //longitude origin
    a = a * K0;
    b = b * K0;

    /* Compute convergence, OS Geodetic Information Paper No 1 */
    var phi = lat * Math.PI / 180.0;
    var c = Math.cos(phi);
    var s = Math.sin(phi);
    var t = Math.tan(phi);
    var t2 = t * t;
    var p = (lon - oDeg) * Math.PI / 180.0;
    var eta2 = a * a * c * c / (b * b) - c * c;

    return (
(p * s) +
(p * p * p * s * c * c * (1.0 + (3.0 * eta2) + (2.0 * eta2 * eta2)) / 3.0) +
(p * p * p * p * p * s * c * c * c * c * (2.0 - t2) / 15.0)
)
* 180.0 / Math.PI;

}

function NGR2NE(ngr) {
    var e;
    var n;

    ngr = ngr.toUpperCase(ngr);

    var bits = ngr.split(' ');
    ngr = "";
    for (var i = 0; i < bits.length; i++)
        ngr += bits[i];

    var c = ngr.charAt(0);
    if (c == 'S') {
        e = 0;
        n = 0;
    }
    else if (c == 'T') {
        e = 500000;
        n = 0;
    }
    else if (c == 'N') {
        n = 500000;
        e = 0;
    }
    else if (c == 'O') {
        n = 500000;
        e = 500000;
    }
    else if (c == 'H') {
        n = 1000000;
        e = 0;
    }
    else
        return null;

    c = ngr.charAt(1);
    if (c == 'I')
        return null;

    c = ngr.charCodeAt(1) - 65;
    if (c > 8)
        c -= 1;
    e += (c % 5) * 100000;
    n += (4 - Math.floor(c / 5)) * 100000;

    c = ngr.substr(2);
    if ((c.length % 2) == 1)
        return null;
    if (c.length > 10)
        return null;

    try {
        var s = c.substr(0, c.length / 2);
        while (s.length < 5)
            s += '0';
        e += parseInt(s, 10);
        if (isNaN(e))
            return null;

        s = c.substr(c.length / 2);
        while (s.length < 5)
            s += '0';
        n += parseInt(s, 10);
        if (isNaN(n))
            return null;

        return new OGBNorthEast(e, n);
    }
    catch (ex) {
        return null;
    }

}