osgridref.js

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/*  Ordnance Survey Grid Reference functions  (c) Chris Veness 2005-2012                          */
/*   - www.movable-type.co.uk/scripts/gridref.js                                                  */
/*   - www.movable-type.co.uk/scripts/latlon-gridref.html                                         */
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/**
 * Creates a OsGridRef object
 *
 * @constructor
 * @param {Number} easting:  Easting in metres from OS false origin
 * @param {Number} northing: Northing in metres from OS false origin
 */
function OsGridRef(easting, northing) {
  this.easting = parseInt(easting, 10);
  this.northing = parseInt(northing, 10);
}


/**
 * Convert (OSGB36) latitude/longitude to Ordnance Survey grid reference easting/northing coordinate
 *
 * @param {LatLon} point: OSGB36 latitude/longitude
 * @return {OsGridRef} OS Grid Reference easting/northing
 *
 * @requires LatLon
 */
OsGridRef.latLongToOsGrid = function(point) {
  var lat = point.lat().toRad(); 
  var lon = point.lon().toRad(); 
  
  var a = 6377563.396, b = 6356256.910;          // Airy 1830 major & minor semi-axes
  var F0 = 0.9996012717;                         // NatGrid scale factor on central meridian
  var lat0 = (49).toRad(), lon0 = (-2).toRad();  // NatGrid true origin is 49ºN,2ºW
  var N0 = -100000, E0 = 400000;                 // northing & easting of true origin, metres
  var e2 = 1 - (b*b)/(a*a);                      // eccentricity squared
  var n = (a-b)/(a+b), n2 = n*n, n3 = n*n*n;

  var cosLat = Math.cos(lat), sinLat = Math.sin(lat);
  var nu = a*F0/Math.sqrt(1-e2*sinLat*sinLat);              // transverse radius of curvature
  var rho = a*F0*(1-e2)/Math.pow(1-e2*sinLat*sinLat, 1.5);  // meridional radius of curvature
  var eta2 = nu/rho-1;

  var Ma = (1 + n + (5/4)*n2 + (5/4)*n3) * (lat-lat0);
  var Mb = (3*n + 3*n*n + (21/8)*n3) * Math.sin(lat-lat0) * Math.cos(lat+lat0);
  var Mc = ((15/8)*n2 + (15/8)*n3) * Math.sin(2*(lat-lat0)) * Math.cos(2*(lat+lat0));
  var Md = (35/24)*n3 * Math.sin(3*(lat-lat0)) * Math.cos(3*(lat+lat0));
  var M = b * F0 * (Ma - Mb + Mc - Md);              // meridional arc

  var cos3lat = cosLat*cosLat*cosLat;
  var cos5lat = cos3lat*cosLat*cosLat;
  var tan2lat = Math.tan(lat)*Math.tan(lat);
  var tan4lat = tan2lat*tan2lat;

  var I = M + N0;
  var II = (nu/2)*sinLat*cosLat;
  var III = (nu/24)*sinLat*cos3lat*(5-tan2lat+9*eta2);
  var IIIA = (nu/720)*sinLat*cos5lat*(61-58*tan2lat+tan4lat);
  var IV = nu*cosLat;
  var V = (nu/6)*cos3lat*(nu/rho-tan2lat);
  var VI = (nu/120) * cos5lat * (5 - 18*tan2lat + tan4lat + 14*eta2 - 58*tan2lat*eta2);

  var dLon = lon-lon0;
  var dLon2 = dLon*dLon, dLon3 = dLon2*dLon, dLon4 = dLon3*dLon, dLon5 = dLon4*dLon, dLon6 = dLon5*dLon;

  var N = I + II*dLon2 + III*dLon4 + IIIA*dLon6;
  var E = E0 + IV*dLon + V*dLon3 + VI*dLon5;

  return new OsGridRef(E, N);
}

/**
 * Convert Ordnance Survey grid reference easting/northing coordinate to (OSGB36) latitude/longitude
 *
 * @param {OsGridRef} easting/northing to be converted to latitude/longitude
 * @return {LatLon} latitude/longitude (in OSGB36) of supplied grid reference
 *
 * @requires LatLon
 */
OsGridRef.osGridToLatLong = function(gridref) {
  var E = gridref.easting;
  var N = gridref.northing;

  var a = 6377563.396, b = 6356256.910;              // Airy 1830 major & minor semi-axes
  var F0 = 0.9996012717;                             // NatGrid scale factor on central meridian
  var lat0 = 49*Math.PI/180, lon0 = -2*Math.PI/180;  // NatGrid true origin
  var N0 = -100000, E0 = 400000;                     // northing & easting of true origin, metres
  var e2 = 1 - (b*b)/(a*a);                          // eccentricity squared
  var n = (a-b)/(a+b), n2 = n*n, n3 = n*n*n;

  var lat=lat0, M=0;
  do {
    lat = (N-N0-M)/(a*F0) + lat;

    var Ma = (1 + n + (5/4)*n2 + (5/4)*n3) * (lat-lat0);
    var Mb = (3*n + 3*n*n + (21/8)*n3) * Math.sin(lat-lat0) * Math.cos(lat+lat0);
    var Mc = ((15/8)*n2 + (15/8)*n3) * Math.sin(2*(lat-lat0)) * Math.cos(2*(lat+lat0));
    var Md = (35/24)*n3 * Math.sin(3*(lat-lat0)) * Math.cos(3*(lat+lat0));
    M = b * F0 * (Ma - Mb + Mc - Md);                // meridional arc

  } while (N-N0-M >= 0.00001);  // ie until < 0.01mm

  var cosLat = Math.cos(lat), sinLat = Math.sin(lat);
  var nu = a*F0/Math.sqrt(1-e2*sinLat*sinLat);              // transverse radius of curvature
  var rho = a*F0*(1-e2)/Math.pow(1-e2*sinLat*sinLat, 1.5);  // meridional radius of curvature
  var eta2 = nu/rho-1;

  var tanLat = Math.tan(lat);
  var tan2lat = tanLat*tanLat, tan4lat = tan2lat*tan2lat, tan6lat = tan4lat*tan2lat;
  var secLat = 1/cosLat;
  var nu3 = nu*nu*nu, nu5 = nu3*nu*nu, nu7 = nu5*nu*nu;
  var VII = tanLat/(2*rho*nu);
  var VIII = tanLat/(24*rho*nu3)*(5+3*tan2lat+eta2-9*tan2lat*eta2);
  var IX = tanLat/(720*rho*nu5)*(61+90*tan2lat+45*tan4lat);
  var X = secLat/nu;
  var XI = secLat/(6*nu3)*(nu/rho+2*tan2lat);
  var XII = secLat/(120*nu5)*(5+28*tan2lat+24*tan4lat);
  var XIIA = secLat/(5040*nu7)*(61+662*tan2lat+1320*tan4lat+720*tan6lat);

  var dE = (E-E0), dE2 = dE*dE, dE3 = dE2*dE, dE4 = dE2*dE2, dE5 = dE3*dE2, dE6 = dE4*dE2, dE7 = dE5*dE2;
  lat = lat - VII*dE2 + VIII*dE4 - IX*dE6;
  var lon = lon0 + X*dE - XI*dE3 + XII*dE5 - XIIA*dE7;
  
  return new LatLon(lat.toDeg(), lon.toDeg());
}


/**
 * Converts standard grid reference ('SU387148') to fully numeric ref ([438700,114800]);
 *   returned co-ordinates are in metres, centred on supplied grid square;
 *   no error-checking is done on gridref (bad input will give bad results or NaN)
 *
 * @param {String} gridref: Standard format OS grid reference
 * @returns {OsGridRef}     Numeric version of grid reference in metres from false origin
 */
OsGridRef.parse = function(gridref) {
  gridref = gridref.trim();
  // get numeric values of letter references, mapping A->0, B->1, C->2, etc:
  var l1 = gridref.toUpperCase().charCodeAt(0) - 'A'.charCodeAt(0);
  var l2 = gridref.toUpperCase().charCodeAt(1) - 'A'.charCodeAt(0);
  // shuffle down letters after 'I' since 'I' is not used in grid:
  if (l1 > 7) l1--;
  if (l2 > 7) l2--;

  // convert grid letters into 100km-square indexes from false origin (grid square SV):
  var e = ((l1-2)%5)*5 + (l2%5);
  var n = (19-Math.floor(l1/5)*5) - Math.floor(l2/5);

  // skip grid letters to get numeric part of ref, stripping any spaces:
  gridref = gridref.slice(2).replace(/ /g,'');

  // append numeric part of references to grid index:
  e += gridref.slice(0, gridref.length/2);
  n += gridref.slice(gridref.length/2);

  // normalise to 1m grid, rounding up to centre of grid square:
  switch (gridref.length) {
    case 0: e += '50000'; n += '50000'; break;
    case 2: e += '5000'; n += '5000'; break;
    case 4: e += '500'; n += '500'; break;
    case 6: e += '50'; n += '50'; break;
    case 8: e += '5'; n += '5'; break;
    // 10-digit refs are already 1m
  }

  return new OsGridRef(e, n);
}

/**
 * Converts this numeric grid reference to standard OS grid reference
 *
 * @param {Number} [digits=6] Precision of returned grid reference (6 digits = metres)
 * @return {String)           This grid reference in standard format
 */
OsGridRef.prototype.toString = function(digits) {
  digits = (typeof digits == 'undefined') ? 10 : digits;
  e = this.easting, n = this.northing;
  
  // get the 100km-grid indices
  var e100k = Math.floor(e/100000), n100k = Math.floor(n/100000);
  
  if (e100k<0 || e100k>6 || n100k<0 || n100k>12) return '';

  // translate those into numeric equivalents of the grid letters
  var l1 = (19-n100k) - (19-n100k)%5 + Math.floor((e100k+10)/5);
  var l2 = (19-n100k)*5%25 + e100k%5;

  // compensate for skipped 'I' and build grid letter-pairs
  if (l1 > 7) l1++;
  if (l2 > 7) l2++;
  var letPair = String.fromCharCode(l1+'A'.charCodeAt(0), l2+'A'.charCodeAt(0));

  // strip 100km-grid indices from easting & northing, and reduce precision
  e = Math.floor((e%100000)/Math.pow(10,5-digits/2));
  n = Math.floor((n%100000)/Math.pow(10,5-digits/2));

  var gridRef = letPair + ' ' + e.padLz(digits/2) + ' ' + n.padLz(digits/2);

  return gridRef;
}

/** Trims whitespace from string (q.v. blog.stevenlevithan.com/archives/faster-trim-javascript) */
if (typeof(String.prototype.trim) === "undefined") {
  String.prototype.trim = function() {
    return this.replace(/^\s\s*/, '').replace(/\s\s*$/, '');
  }
}

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