Random locations nearby specified location limited by radius

geo-points-distribute-nearby.js

/**
 * Get random geo point [latitude, longitude] within some distance from specified geo point (lat, long)
 * 
 * Points will be uniformly-distributed on multiple calls.
 *
 * → Demo is here: http://jsfiddle.net/siberex/f72ffn1x/
 * 
 * @param lat Latitude in degrees
 * @param lng Longitude in degrees
 * @param distance Distance in meters to limit point distribution to.
 *                 If negative value is provided, points will be generated on exact distance (e.g. on circle border).
 * @returns {*[]} Array with [latitude, longitude]
 * 
 * @author Stepan Legachev <me@sib.li>
 */
exports.getRandomLocation = function(lat, lng, distance = 10000) {
    // Convert to radians
    lat *= Math.PI / 180;
    lng *= Math.PI / 180;

    var radius;

    // Distance should be set in meters, negative for exact distance
    if (distance < 0) {
        // Exact distance
        radius = Math.abs(distance);
    } else {
        // Get uniformly-random distribution within peovided distance
        // http://stackoverflow.com/questions/5837572/generate-a-random-point-within-a-circle-uniformly
        radius = Math.random() + Math.random();
        radius = radius > 1 ? 2 - radius : radius;
        radius *= distance ? distance : 10000; // multiply by distance meters
    }

    // Convert radius from meters to degrees to radians
    // 111319.9 meters = one degree along the equator
    radius /= 111319.9;
    // Correction for the actual distance from equator is NOT needed here
    // radius *= Math.cos(lat);
    // Convert to radians
    radius *= Math.PI / 180;

    // Random angle
    var angle = Math.random() * Math.PI * 2;

    // Get a point {nLat,nLng} in a distance={radius} out on the {angle} radial from point {lat,lng}
    // http://williams.best.vwh.net/avform.htm#LL
    var nLng,
        nLat = Math.asin( Math.sin(lat) * Math.cos(radius) + Math.cos(lat) * Math.sin(radius) * Math.cos(angle) );
    if (Math.cos(nLat) == 0) {
        nLng = lng;
    } else {
        nLng = (lng - Math.asin( Math.sin(angle) * Math.sin(radius) / Math.cos(nLat) ) + Math.PI) % (Math.PI * 2) - Math.PI
    }

    // Convert to degrees
    nLat *= 180 / Math.PI;
    nLng *= 180 / Math.PI;

    return [nLat, nLng];
};