latlon-spherical.js

/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
/* Latitude/longitude spherical geodesy tools                         (c) Chris Veness 2002-2017  */
/*                                                                                   MIT Licence  */
/* www.movable-type.co.uk/scripts/latlong.html                                                    */
/* www.movable-type.co.uk/scripts/geodesy/docs/module-latlon-spherical.html                       */
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */

'use strict';
if (typeof module!='undefined' && module.exports) var Dms = require('./dms'); // ≡ import Dms from 'dms.js'


/**
 * Library of geodesy functions for operations on a spherical earth model.
 *
 * @module   latlon-spherical
 * @requires dms
 */


/**
 * Creates a LatLon point on the earth's surface at the specified latitude / longitude.
 *
 * @constructor
 * @param {number} lat - Latitude in degrees.
 * @param {number} lon - Longitude in degrees.
 *
 * @example
 *     var p1 = new LatLon(52.205, 0.119);
 */
function LatLon(lat, lon) {
    // allow instantiation without 'new'
    if (!(this instanceof LatLon)) return new LatLon(lat, lon);

    this.lat = Number(lat);
    this.lon = Number(lon);
}


/**
 * Returns the distance from ‘this’ point to destination point (using haversine formula).
 *
 * @param   {LatLon} point - Latitude/longitude of destination point.
 * @param   {number} [radius=6371e3] - (Mean) radius of earth (defaults to radius in metres).
 * @returns {number} Distance between this point and destination point, in same units as radius.
 *
 * @example
 *     var p1 = new LatLon(52.205, 0.119);
 *     var p2 = new LatLon(48.857, 2.351);
 *     var d = p1.distanceTo(p2); // 404.3 km
 */
LatLon.prototype.distanceTo = function(point, radius) {
    if (!(point instanceof LatLon)) throw new TypeError('point is not LatLon object');
    radius = (radius === undefined) ? 6371e3 : Number(radius);

    var R = radius;
    var φ1 = this.lat.toRadians(),  λ1 = this.lon.toRadians();
    var φ2 = point.lat.toRadians(), λ2 = point.lon.toRadians();
    var Δφ = φ2 - φ1;
    var Δλ = λ2 - λ1;

    var a = Math.sin(Δφ/2) * Math.sin(Δφ/2)
          + Math.cos(φ1) * Math.cos(φ2)
          * Math.sin(Δλ/2) * Math.sin(Δλ/2);
    var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
    var d = R * c;

    return d;
};


/**
 * Returns the (initial) bearing from ‘this’ point to destination point.
 *
 * @param   {LatLon} point - Latitude/longitude of destination point.
 * @returns {number} Initial bearing in degrees from north.
 *
 * @example
 *     var p1 = new LatLon(52.205, 0.119);
 *     var p2 = new LatLon(48.857, 2.351);
 *     var b1 = p1.bearingTo(p2); // 156.2°
 */
LatLon.prototype.bearingTo = function(point) {
    if (!(point instanceof LatLon)) throw new TypeError('point is not LatLon object');

    var φ1 = this.lat.toRadians(), φ2 = point.lat.toRadians();
    var Δλ = (point.lon-this.lon).toRadians();

    // see http://mathforum.org/library/drmath/view/55417.html
    var y = Math.sin(Δλ) * Math.cos(φ2);
    var x = Math.cos(φ1)*Math.sin(φ2) -
            Math.sin(φ1)*Math.cos(φ2)*Math.cos(Δλ);
    var θ = Math.atan2(y, x);

    return (θ.toDegrees()+360) % 360;
};


/**
 * Returns final bearing arriving at destination destination point from ‘this’ point; the final bearing
 * will differ from the initial bearing by varying degrees according to distance and latitude.
 *
 * @param   {LatLon} point - Latitude/longitude of destination point.
 * @returns {number} Final bearing in degrees from north.
 *
 * @example
 *     var p1 = new LatLon(52.205, 0.119);
 *     var p2 = new LatLon(48.857, 2.351);
 *     var b2 = p1.finalBearingTo(p2); // 157.9°
 */
LatLon.prototype.finalBearingTo = function(point) {
    if (!(point instanceof LatLon)) throw new TypeError('point is not LatLon object');

    // get initial bearing from destination point to this point & reverse it by adding 180°
    return ( point.bearingTo(this)+180 ) % 360;
};


/**
 * Returns the midpoint between ‘this’ point and the supplied point.
 *
 * @param   {LatLon} point - Latitude/longitude of destination point.
 * @returns {LatLon} Midpoint between this point and the supplied point.
 *
 * @example
 *     var p1 = new LatLon(52.205, 0.119);
 *     var p2 = new LatLon(48.857, 2.351);
 *     var pMid = p1.midpointTo(p2); // 50.5363°N, 001.2746°E
 */
LatLon.prototype.midpointTo = function(point) {
    if (!(point instanceof LatLon)) throw new TypeError('point is not LatLon object');

    // φm = atan2( sinφ1 + sinφ2, √( (cosφ1 + cosφ2⋅cosΔλ) ⋅ (cosφ1 + cosφ2⋅cosΔλ) ) + cos²φ2⋅sin²Δλ )
    // λm = λ1 + atan2(cosφ2⋅sinΔλ, cosφ1 + cosφ2⋅cosΔλ)
    // see http://mathforum.org/library/drmath/view/51822.html for derivation

    var φ1 = this.lat.toRadians(), λ1 = this.lon.toRadians();
    var φ2 = point.lat.toRadians();
    var Δλ = (point.lon-this.lon).toRadians();

    var Bx = Math.cos(φ2) * Math.cos(Δλ);
    var By = Math.cos(φ2) * Math.sin(Δλ);

    var x = Math.sqrt((Math.cos(φ1) + Bx) * (Math.cos(φ1) + Bx) + By * By);
    var y = Math.sin(φ1) + Math.sin(φ2);
    var φ3 = Math.atan2(y, x);

    var λ3 = λ1 + Math.atan2(By, Math.cos(φ1) + Bx);

    return new LatLon(φ3.toDegrees(), (λ3.toDegrees()+540)%360-180); // normalise to −180..+180°
};


/**
 * Returns the point at given fraction between ‘this’ point and specified point.
 *
 * @param   {LatLon} point - Latitude/longitude of destination point.
 * @param   {number} fraction - Fraction between the two points (0 = this point, 1 = specified point).
 * @returns {LatLon} Intermediate point between this point and destination point.
 *
 * @example
 *   let p1 = new LatLon(52.205, 0.119);
 *   let p2 = new LatLon(48.857, 2.351);
 *   let pMid = p1.intermediatePointTo(p2, 0.25); // 51.3721°N, 000.7073°E
 */
LatLon.prototype.intermediatePointTo = function(point, fraction) {
    if (!(point instanceof LatLon)) throw new TypeError('point is not LatLon object');

    var φ1 = this.lat.toRadians(), λ1 = this.lon.toRadians();
    var φ2 = point.lat.toRadians(), λ2 = point.lon.toRadians();
    var sinφ1 = Math.sin(φ1), cosφ1 = Math.cos(φ1), sinλ1 = Math.sin(λ1), cosλ1 = Math.cos(λ1);
    var sinφ2 = Math.sin(φ2), cosφ2 = Math.cos(φ2), sinλ2 = Math.sin(λ2), cosλ2 = Math.cos(λ2);

    // distance between points
    var Δφ = φ2 - φ1;
    var Δλ = λ2 - λ1;
    var a = Math.sin(Δφ/2) * Math.sin(Δφ/2)
        + Math.cos(φ1) * Math.cos(φ2) * Math.sin(Δλ/2) * Math.sin(Δλ/2);
    var δ = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));

    var A = Math.sin((1-fraction)*δ) / Math.sin(δ);
    var B = Math.sin(fraction*δ) / Math.sin(δ);

    var x = A * cosφ1 * cosλ1 + B * cosφ2 * cosλ2;
    var y = A * cosφ1 * sinλ1 + B * cosφ2 * sinλ2;
    var z = A * sinφ1 + B * sinφ2;

    var φ3 = Math.atan2(z, Math.sqrt(x*x + y*y));
    var λ3 = Math.atan2(y, x);

    return new LatLon(φ3.toDegrees(), (λ3.toDegrees()+540)%360-180); // normalise lon to −180..+180°
};


/**
 * Returns the destination point from ‘this’ point having travelled the given distance on the
 * given initial bearing (bearing normally varies around path followed).
 *
 * @param   {number} distance - Distance travelled, in same units as earth radius (default: metres).
 * @param   {number} bearing - Initial bearing in degrees from north.
 * @param   {number} [radius=6371e3] - (Mean) radius of earth (defaults to radius in metres).
 * @returns {LatLon} Destination point.
 *
 * @example
 *     var p1 = new LatLon(51.4778, -0.0015);
 *     var p2 = p1.destinationPoint(7794, 300.7); // 51.5135°N, 000.0983°W
 */
LatLon.prototype.destinationPoint = function(distance, bearing, radius) {
    radius = (radius === undefined) ? 6371e3 : Number(radius);

    // sinφ2 = sinφ1⋅cosδ + cosφ1⋅sinδ⋅cosθ
    // tanΔλ = sinθ⋅sinδ⋅cosφ1 / cosδ−sinφ1⋅sinφ2
    // see www.edwilliams.org/avform.htm#LL

    var δ = Number(distance) / radius; // angular distance in radians
    var θ = Number(bearing).toRadians();

    var φ1 = this.lat.toRadians();
    var λ1 = this.lon.toRadians();

    var sinφ1 = Math.sin(φ1), cosφ1 = Math.cos(φ1);
    var sinδ = Math.sin(δ), cosδ = Math.cos(δ);
    var sinθ = Math.sin(θ), cosθ = Math.cos(θ);

    var sinφ2 = sinφ1*cosδ + cosφ1*sinδ*cosθ;
    var φ2 = Math.asin(sinφ2);
    var y = sinθ * sinδ * cosφ1;
    var x = cosδ - sinφ1 * sinφ2;
    var λ2 = λ1 + Math.atan2(y, x);

    return new LatLon(φ2.toDegrees(), (λ2.toDegrees()+540)%360-180); // normalise to −180..+180°
};


/**
 * Returns the point of intersection of two paths defined by point and bearing.
 *
 * @param   {LatLon} p1 - First point.
 * @param   {number} brng1 - Initial bearing from first point.
 * @param   {LatLon} p2 - Second point.
 * @param   {number} brng2 - Initial bearing from second point.
 * @returns {LatLon|null} Destination point (null if no unique intersection defined).
 *
 * @example
 *     var p1 = LatLon(51.8853, 0.2545), brng1 = 108.547;
 *     var p2 = LatLon(49.0034, 2.5735), brng2 =  32.435;
 *     var pInt = LatLon.intersection(p1, brng1, p2, brng2); // 50.9078°N, 004.5084°E
 */
LatLon.intersection = function(p1, brng1, p2, brng2) {
    if (!(p1 instanceof LatLon)) throw new TypeError('p1 is not LatLon object');
    if (!(p2 instanceof LatLon)) throw new TypeError('p2 is not LatLon object');

    // see www.edwilliams.org/avform.htm#Intersection

    var φ1 = p1.lat.toRadians(), λ1 = p1.lon.toRadians();
    var φ2 = p2.lat.toRadians(), λ2 = p2.lon.toRadians();
    var θ13 = Number(brng1).toRadians(), θ23 = Number(brng2).toRadians();
    var Δφ = φ2-φ1, Δλ = λ2-λ1;

    // angular distance p1-p2
    var δ12 = 2*Math.asin( Math.sqrt( Math.sin(Δφ/2)*Math.sin(Δφ/2)
        + Math.cos(φ1)*Math.cos(φ2)*Math.sin(Δλ/2)*Math.sin(Δλ/2) ) );
    if (δ12 == 0) return null;

    // initial/final bearings between points
    var θa = Math.acos( ( Math.sin(φ2) - Math.sin(φ1)*Math.cos(δ12) ) / ( Math.sin(δ12)*Math.cos(φ1) ) );
    if (isNaN(θa)) θa = 0; // protect against rounding
    var θb = Math.acos( ( Math.sin(φ1) - Math.sin(φ2)*Math.cos(δ12) ) / ( Math.sin(δ12)*Math.cos(φ2) ) );

    var θ12 = Math.sin(λ2-λ1)>0 ? θa : 2*Math.PI-θa;
    var θ21 = Math.sin(λ2-λ1)>0 ? 2*Math.PI-θb : θb;

    var α1 = θ13 - θ12; // angle 2-1-3
    var α2 = θ21 - θ23; // angle 1-2-3

    if (Math.sin(α1)==0 && Math.sin(α2)==0) return null; // infinite intersections
    if (Math.sin(α1)*Math.sin(α2) < 0) return null;      // ambiguous intersection

    var α3 = Math.acos( -Math.cos(α1)*Math.cos(α2) + Math.sin(α1)*Math.sin(α2)*Math.cos(δ12) );
    var δ13 = Math.atan2( Math.sin(δ12)*Math.sin(α1)*Math.sin(α2), Math.cos(α2)+Math.cos(α1)*Math.cos(α3) );
    var φ3 = Math.asin( Math.sin(φ1)*Math.cos(δ13) + Math.cos(φ1)*Math.sin(δ13)*Math.cos(θ13) );
    var Δλ13 = Math.atan2( Math.sin(θ13)*Math.sin(δ13)*Math.cos(φ1), Math.cos(δ13)-Math.sin(φ1)*Math.sin(φ3) );
    var λ3 = λ1 + Δλ13;

    return new LatLon(φ3.toDegrees(), (λ3.toDegrees()+540)%360-180); // normalise to −180..+180°
};


/**
 * Returns (signed) distance from ‘this’ point to great circle defined by start-point and end-point.
 *
 * @param   {LatLon} pathStart - Start point of great circle path.
 * @param   {LatLon} pathEnd - End point of great circle path.
 * @param   {number} [radius=6371e3] - (Mean) radius of earth (defaults to radius in metres).
 * @returns {number} Distance to great circle (-ve if to left, +ve if to right of path).
 *
 * @example
 *   var pCurrent = new LatLon(53.2611, -0.7972);
 *   var p1 = new LatLon(53.3206, -1.7297);
 *   var p2 = new LatLon(53.1887,  0.1334);
 *   var d = pCurrent.crossTrackDistanceTo(p1, p2);  // -307.5 m
 */
LatLon.prototype.crossTrackDistanceTo = function(pathStart, pathEnd, radius) {
    if (!(pathStart instanceof LatLon)) throw new TypeError('pathStart is not LatLon object');
    if (!(pathEnd instanceof LatLon)) throw new TypeError('pathEnd is not LatLon object');
    var R = (radius === undefined) ? 6371e3 : Number(radius);

    var δ13 = pathStart.distanceTo(this, R) / R;
    var θ13 = pathStart.bearingTo(this).toRadians();
    var θ12 = pathStart.bearingTo(pathEnd).toRadians();

    var δxt = Math.asin(Math.sin(δ13) * Math.sin(θ13-θ12));

    return δxt * R;
};


/**
 * Returns how far ‘this’ point is along a path from from start-point, heading towards end-point.
 * That is, if a perpendicular is drawn from ‘this’ point to the (great circle) path, the along-track
 * distance is the distance from the start point to where the perpendicular crosses the path.
 *
 * @param   {LatLon} pathStart - Start point of great circle path.
 * @param   {LatLon} pathEnd - End point of great circle path.
 * @param   {number} [radius=6371e3] - (Mean) radius of earth (defaults to radius in metres).
 * @returns {number} Distance along great circle to point nearest ‘this’ point.
 *
 * @example
 *   var pCurrent = new LatLon(53.2611, -0.7972);
 *   var p1 = new LatLon(53.3206, -1.7297);
 *   var p2 = new LatLon(53.1887,  0.1334);
 *   var d = pCurrent.alongTrackDistanceTo(p1, p2);  // 62.331 km
 */
LatLon.prototype.alongTrackDistanceTo = function(pathStart, pathEnd, radius) {
    if (!(pathStart instanceof LatLon)) throw new TypeError('pathStart is not LatLon object');
    if (!(pathEnd instanceof LatLon)) throw new TypeError('pathEnd is not LatLon object');
    var R = (radius === undefined) ? 6371e3 : Number(radius);

    var δ13 = pathStart.distanceTo(this, R) / R;
    var θ13 = pathStart.bearingTo(this).toRadians();
    var θ12 = pathStart.bearingTo(pathEnd).toRadians();

    var δxt = Math.asin(Math.sin(δ13) * Math.sin(θ13-θ12));

    var δat = Math.acos(Math.cos(δ13) / Math.abs(Math.cos(δxt)));

    return δat*Math.sign(Math.cos(θ12-θ13)) * R;
};


/**
 * Returns maximum latitude reached when travelling on a great circle on given bearing from this
 * point ('Clairaut's formula'). Negate the result for the minimum latitude (in the Southern
 * hemisphere).
 *
 * The maximum latitude is independent of longitude; it will be the same for all points on a given
 * latitude.
 *
 * @param {number} bearing - Initial bearing.
 * @param {number} latitude - Starting latitude.
 */
LatLon.prototype.maxLatitude = function(bearing) {
    var θ = Number(bearing).toRadians();

    var φ = this.lat.toRadians();

    var φMax = Math.acos(Math.abs(Math.sin(θ)*Math.cos(φ)));

    return φMax.toDegrees();
};


/**
 * Returns the pair of meridians at which a great circle defined by two points crosses the given
 * latitude. If the great circle doesn't reach the given latitude, null is returned.
 *
 * @param {LatLon} point1 - First point defining great circle.
 * @param {LatLon} point2 - Second point defining great circle.
 * @param {number} latitude - Latitude crossings are to be determined for.
 * @returns {Object|null} Object containing { lon1, lon2 } or null if given latitude not reached.
 */
LatLon.crossingParallels = function(point1, point2, latitude) {
    var φ = Number(latitude).toRadians();

    var φ1 = point1.lat.toRadians();
    var λ1 = point1.lon.toRadians();
    var φ2 = point2.lat.toRadians();
    var λ2 = point2.lon.toRadians();

    var Δλ = λ2 - λ1;

    var x = Math.sin(φ1) * Math.cos(φ2) * Math.cos(φ) * Math.sin(Δλ);
    var y = Math.sin(φ1) * Math.cos(φ2) * Math.cos(φ) * Math.cos(Δλ) - Math.cos(φ1) * Math.sin(φ2) * Math.cos(φ);
    var z = Math.cos(φ1) * Math.cos(φ2) * Math.sin(φ) * Math.sin(Δλ);

    if (z*z > x*x + y*y) return null; // great circle doesn't reach latitude

    var λm = Math.atan2(-y, x);                  // longitude at max latitude
    var Δλi = Math.acos(z / Math.sqrt(x*x+y*y)); // Δλ from λm to intersection points

    var λi1 = λ1 + λm - Δλi;
    var λi2 = λ1 + λm + Δλi;

    return { lon1: (λi1.toDegrees()+540)%360-180, lon2: (λi2.toDegrees()+540)%360-180 }; // normalise to −180..+180°
};


/* Rhumb - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */

/**
 * Returns the distance travelling from ‘this’ point to destination point along a rhumb line.
 *
 * @param   {LatLon} point - Latitude/longitude of destination point.
 * @param   {number} [radius=6371e3] - (Mean) radius of earth (defaults to radius in metres).
 * @returns {number} Distance in km between this point and destination point (same units as radius).
 *
 * @example
 *     var p1 = new LatLon(51.127, 1.338);
 *     var p2 = new LatLon(50.964, 1.853);
 *     var d = p1.distanceTo(p2); // 40.31 km
 */
LatLon.prototype.rhumbDistanceTo = function(point, radius) {
    if (!(point instanceof LatLon)) throw new TypeError('point is not LatLon object');
    radius = (radius === undefined) ? 6371e3 : Number(radius);

    // see www.edwilliams.org/avform.htm#Rhumb

    var R = radius;
    var φ1 = this.lat.toRadians(), φ2 = point.lat.toRadians();
    var Δφ = φ2 - φ1;
    var Δλ = Math.abs(point.lon-this.lon).toRadians();
    // if dLon over 180° take shorter rhumb line across the anti-meridian:
    if (Δλ > Math.PI) Δλ -= 2*Math.PI;

    // on Mercator projection, longitude distances shrink by latitude; q is the 'stretch factor'
    // q becomes ill-conditioned along E-W line (0/0); use empirical tolerance to avoid it
    var Δψ = Math.log(Math.tan(φ2/2+Math.PI/4)/Math.tan(φ1/2+Math.PI/4));
    var q = Math.abs(Δψ) > 10e-12 ? Δφ/Δψ : Math.cos(φ1);

    // distance is pythagoras on 'stretched' Mercator projection
    var δ = Math.sqrt(Δφ*Δφ + q*q*Δλ*Δλ); // angular distance in radians
    var dist = δ * R;

    return dist;
};


/**
 * Returns the bearing from ‘this’ point to destination point along a rhumb line.
 *
 * @param   {LatLon} point - Latitude/longitude of destination point.
 * @returns {number} Bearing in degrees from north.
 *
 * @example
 *     var p1 = new LatLon(51.127, 1.338);
 *     var p2 = new LatLon(50.964, 1.853);
 *     var d = p1.rhumbBearingTo(p2); // 116.7 m
 */
LatLon.prototype.rhumbBearingTo = function(point) {
    if (!(point instanceof LatLon)) throw new TypeError('point is not LatLon object');

    var φ1 = this.lat.toRadians(), φ2 = point.lat.toRadians();
    var Δλ = (point.lon-this.lon).toRadians();
    // if dLon over 180° take shorter rhumb line across the anti-meridian:
    if (Δλ >  Math.PI) Δλ -= 2*Math.PI;
    if (Δλ < -Math.PI) Δλ += 2*Math.PI;

    var Δψ = Math.log(Math.tan(φ2/2+Math.PI/4)/Math.tan(φ1/2+Math.PI/4));

    var θ = Math.atan2(Δλ, Δψ);

    return (θ.toDegrees()+360) % 360;
};


/**
 * Returns the destination point having travelled along a rhumb line from ‘this’ point the given
 * distance on the  given bearing.
 *
 * @param   {number} distance - Distance travelled, in same units as earth radius (default: metres).
 * @param   {number} bearing - Bearing in degrees from north.
 * @param   {number} [radius=6371e3] - (Mean) radius of earth (defaults to radius in metres).
 * @returns {LatLon} Destination point.
 *
 * @example
 *     var p1 = new LatLon(51.127, 1.338);
 *     var p2 = p1.rhumbDestinationPoint(40300, 116.7); // 50.9642°N, 001.8530°E
 */
LatLon.prototype.rhumbDestinationPoint = function(distance, bearing, radius) {
    radius = (radius === undefined) ? 6371e3 : Number(radius);

    var δ = Number(distance) / radius; // angular distance in radians
    var φ1 = this.lat.toRadians(), λ1 = this.lon.toRadians();
    var θ = Number(bearing).toRadians();

    var Δφ = δ * Math.cos(θ);
    var φ2 = φ1 + Δφ;

    // check for some daft bugger going past the pole, normalise latitude if so
    if (Math.abs(φ2) > Math.PI/2) φ2 = φ2>0 ? Math.PI-φ2 : -Math.PI-φ2;

    var Δψ = Math.log(Math.tan(φ2/2+Math.PI/4)/Math.tan(φ1/2+Math.PI/4));
    var q = Math.abs(Δψ) > 10e-12 ? Δφ / Δψ : Math.cos(φ1); // E-W course becomes ill-conditioned with 0/0

    var Δλ = δ*Math.sin(θ)/q;
    var λ2 = λ1 + Δλ;

    return new LatLon(φ2.toDegrees(), (λ2.toDegrees()+540) % 360 - 180); // normalise to −180..+180°
};


/**
 * Returns the loxodromic midpoint (along a rhumb line) between ‘this’ point and second point.
 *
 * @param   {LatLon} point - Latitude/longitude of second point.
 * @returns {LatLon} Midpoint between this point and second point.
 *
 * @example
 *     var p1 = new LatLon(51.127, 1.338);
 *     var p2 = new LatLon(50.964, 1.853);
 *     var pMid = p1.rhumbMidpointTo(p2); // 51.0455°N, 001.5957°E
 */
LatLon.prototype.rhumbMidpointTo = function(point) {
    if (!(point instanceof LatLon)) throw new TypeError('point is not LatLon object');

    // http://mathforum.org/kb/message.jspa?messageID=148837

    var φ1 = this.lat.toRadians(), λ1 = this.lon.toRadians();
    var φ2 = point.lat.toRadians(), λ2 = point.lon.toRadians();

    if (Math.abs(λ2-λ1) > Math.PI) λ1 += 2*Math.PI; // crossing anti-meridian

    var φ3 = (φ1+φ2)/2;
    var f1 = Math.tan(Math.PI/4 + φ1/2);
    var f2 = Math.tan(Math.PI/4 + φ2/2);
    var f3 = Math.tan(Math.PI/4 + φ3/2);
    var λ3 = ( (λ2-λ1)*Math.log(f3) + λ1*Math.log(f2) - λ2*Math.log(f1) ) / Math.log(f2/f1);

    if (!isFinite(λ3)) λ3 = (λ1+λ2)/2; // parallel of latitude

    var p = LatLon(φ3.toDegrees(), (λ3.toDegrees()+540)%360-180); // normalise to −180..+180°

    return p;
};


/* Area - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */


/**
 * Calculates the area of a spherical polygon where the sides of the polygon are great circle
 * arcs joining the vertices.
 *
 * @param   {LatLon[]} polygon - Array of points defining vertices of the polygon
 * @param   {number} [radius=6371e3] - (Mean) radius of earth (defaults to radius in metres).
 * @returns {number} The area of the polygon, in the same units as radius.
 *
 * @example
 *   var polygon = [new LatLon(0,0), new LatLon(1,0), new LatLon(0,1)];
 *   var area = LatLon.areaOf(polygon); // 6.18e9 m²
 */
LatLon.areaOf = function(polygon, radius) {
    // uses method due to Karney: osgeo-org.1560.x6.nabble.com/Area-of-a-spherical-polygon-td3841625.html;
    // for each edge of the polygon, tan(E/2) = tan(Δλ/2)·(tan(φ1/2) + tan(φ2/2)) / (1 + tan(φ1/2)·tan(φ2/2))
    // where E is the spherical excess of the trapezium obtained by extending the edge to the equator

    var R = (radius === undefined) ? 6371e3 : Number(radius);

    // close polygon so that last point equals first point
    var closed = polygon[0].equals(polygon[polygon.length-1]);
    if (!closed) polygon.push(polygon[0]);

    var nVertices = polygon.length - 1;

    var S = 0; // spherical excess in steradians
    for (var v=0; v<nVertices; v++) {
        var φ1 = polygon[v].lat.toRadians();
        var φ2 = polygon[v+1].lat.toRadians();
        var Δλ = (polygon[v+1].lon - polygon[v].lon).toRadians();
        var E = 2 * Math.atan2(Math.tan(Δλ/2) * (Math.tan(φ1/2)+Math.tan(φ2/2)), 1 + Math.tan(φ1/2)*Math.tan(φ2/2));
        S += E;
    }

    if (isPoleEnclosedBy(polygon)) S = Math.abs(S) - 2*Math.PI;

    var A = Math.abs(S * R*R); // area in units of R

    if (!closed) polygon.pop(); // restore polygon to pristine condition

    return A;

    // returns whether polygon encloses pole: sum of course deltas around pole is 0° rather than
    // normal ±360°: blog.element84.com/determining-if-a-spherical-polygon-contains-a-pole.html
    function isPoleEnclosedBy(polygon) {
        // TODO: any better test than this?
        var ΣΔ = 0;
        var prevBrng = polygon[0].bearingTo(polygon[1]);
        for (var v=0; v<polygon.length-1; v++) {
            var initBrng = polygon[v].bearingTo(polygon[v+1]);
            var finalBrng = polygon[v].finalBearingTo(polygon[v+1]);
            ΣΔ += (initBrng - prevBrng + 540) % 360 - 180;
            ΣΔ += (finalBrng - initBrng + 540) % 360 - 180;
            prevBrng = finalBrng;
        }
        var initBrng = polygon[0].bearingTo(polygon[1]);
        ΣΔ += (initBrng - prevBrng + 540) % 360 - 180;
        // TODO: fix (intermittant) edge crossing pole - eg (85,90), (85,0), (85,-90)
        var enclosed = Math.abs(ΣΔ) < 90; // 0°-ish
        return enclosed;
    }
};


/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */


/**
 * Checks if another point is equal to ‘this’ point.
 *
 * @param   {LatLon} point - Point to be compared against this point.
 * @returns {bool}   True if points are identical.
 *
 * @example
 *   var p1 = new LatLon(52.205, 0.119);
 *   var p2 = new LatLon(52.205, 0.119);
 *   var equal = p1.equals(p2); // true
 */
LatLon.prototype.equals = function(point) {
    if (!(point instanceof LatLon)) throw new TypeError('point is not LatLon object');

    if (this.lat != point.lat) return false;
    if (this.lon != point.lon) return false;

    return true;
};


/**
 * Returns a string representation of ‘this’ point, formatted as degrees, degrees+minutes, or
 * degrees+minutes+seconds.
 *
 * @param   {string} [format=dms] - Format point as 'd', 'dm', 'dms'.
 * @param   {number} [dp=0|2|4] - Number of decimal places to use - default 0 for dms, 2 for dm, 4 for d.
 * @returns {string} Comma-separated latitude/longitude.
 */
LatLon.prototype.toString = function(format, dp) {
    return Dms.toLat(this.lat, format, dp) + ', ' + Dms.toLon(this.lon, format, dp);
};


/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */

/** Extend Number object with method to convert numeric degrees to radians */
if (Number.prototype.toRadians === undefined) {
    Number.prototype.toRadians = function() { return this * Math.PI / 180; };
}

/** Extend Number object with method to convert radians to numeric (signed) degrees */
if (Number.prototype.toDegrees === undefined) {
    Number.prototype.toDegrees = function() { return this * 180 / Math.PI; };
}

/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
if (typeof module != 'undefined' && module.exports) module.exports = LatLon; // ≡ export default LatLon