使用说明

WGS-84：是国际标准，GPS坐标（Google Earth使用、或者GPS模块）
GCJ-02：中国坐标偏移标准，Google Map、高德、腾讯使用
BD-09：百度坐标偏移标准，Baidu Map使用

//WGS-84 to GCJ-02
GPS.gcj_encrypt();

//GCJ-02 to WGS-84 粗略
GPS.gcj_decrypt();

//GCJ-02 to WGS-84 精确(二分极限法)
// var threshold = 0.000000001; 目前设置的是精确到小数点后9位，这个值越小，越精确，但是javascript中，浮点运算本身就不太精确，九位在GPS里也偏差不大了
GSP.gcj_decrypt_exact();

//GCJ-02 to BD-09
GPS.bd_encrypt();

//BD-09 to GCJ-02
GPS.bd_decrypt();

//求距离
GPS.distance();

代码

var GPS = {
    PI: 3.14159265358979324,
    x_pi: 3.14159265358979324 * 3000.0 / 180.0,
    delta: function(lat, lon) {
        // Krasovsky 1940
        //
        // a = 6378245.0, 1/f = 298.3
        // b = a * (1 - f)
        // ee = (a^2 - b^2) / a^2;
        var a = 6378245.0;
        var ee = 0.00669342162296594323;
        var dLat = this.transformLat(lon - 105.0, lat - 35.0);
        var dLon = this.transformLon(lon - 105.0, lat - 35.0);
        var radLat = lat / 180.0 * this.PI;
        var magic = Math.sin(radLat);
        magic = 1 - ee * magic * magic;
        var sqrtMagic = Math.sqrt(magic);
        dLat = (dLat * 180.0) / ((a * (1 - ee)) / (magic * sqrtMagic) * this.PI);
        dLon = (dLon * 180.0) / (a / sqrtMagic * Math.cos(radLat) * this.PI);
        return {
            'lat': dLat,
            'lon': dLon
        };
    },
    //WGS-84 to GCJ-02
    gcj_encrypt: function(wgsLat, wgsLon) {
        if (this.outOfChina(wgsLat, wgsLon))
            return {
                'lat': wgsLat,
                'lon': wgsLon
            };

        var d = this.delta(wgsLat, wgsLon);
        return {
            'lat': wgsLat + d.lat,
            'lon': wgsLon + d.lon
        };
    },
    //GCJ-02 to WGS-84
    gcj_decrypt: function(gcjLat, gcjLon) {
        if (this.outOfChina(gcjLat, gcjLon))
            return {
                'lat': gcjLat,
                'lon': gcjLon
            };

        var d = this.delta(gcjLat, gcjLon);
        return {
            'lat': gcjLat - d.lat,
            'lon': gcjLon - d.lon
        };
    },
    //GCJ-02 to WGS-84 exactly
    gcj_decrypt_exact: function(gcjLat, gcjLon) {
        var initDelta = 0.01;
        var threshold = 0.000000001;
        var dLat = initDelta,
            dLon = initDelta;
        var mLat = gcjLat - dLat,
            mLon = gcjLon - dLon;
        var pLat = gcjLat + dLat,
            pLon = gcjLon + dLon;
        var wgsLat, wgsLon, i = 0;
        while (1) {
            wgsLat = (mLat + pLat) / 2;
            wgsLon = (mLon + pLon) / 2;
            var tmp = this.gcj_encrypt(wgsLat, wgsLon)
            dLat = tmp.lat - gcjLat;
            dLon = tmp.lon - gcjLon;
            if ((Math.abs(dLat) < threshold) && (Math.abs(dLon) < threshold))
                break;

            if (dLat > 0) pLat = wgsLat;
            else mLat = wgsLat;
            if (dLon > 0) pLon = wgsLon;
            else mLon = wgsLon;

            if (++i > 10000) break;
        }
        //console.log(i);
        return {
            'lat': wgsLat,
            'lon': wgsLon
        };
    },
    //GCJ-02 to BD-09
    bd_encrypt: function(gcjLat, gcjLon) {
        var x = gcjLon,
            y = gcjLat;
        var z = Math.sqrt(x * x + y * y) + 0.00002 * Math.sin(y * this.x_pi);
        var theta = Math.atan2(y, x) + 0.000003 * Math.cos(x * this.x_pi);
        bdLon = z * Math.cos(theta) + 0.0065;
        bdLat = z * Math.sin(theta) + 0.006;
        return {
            'lat': bdLat,
            'lon': bdLon
        };
    },
    //BD-09 to GCJ-02
    bd_decrypt: function(bdLat, bdLon) {
        var x = bdLon - 0.0065,
            y = bdLat - 0.006;
        var z = Math.sqrt(x * x + y * y) - 0.00002 * Math.sin(y * this.x_pi);
        var theta = Math.atan2(y, x) - 0.000003 * Math.cos(x * this.x_pi);
        var gcjLon = z * Math.cos(theta);
        var gcjLat = z * Math.sin(theta);
        return {
            'lat': gcjLat,
            'lon': gcjLon
        };
    },
    distance: function(latA, logA, latB, logB) {
        var earthR = 6371000;
        var x = Math.cos(latA * Math.PI / 180) * Math.cos(latB * Math.PI / 180) * Math.cos((logA - logB) * Math.PI / 180);
        var y = Math.sin(latA * Math.PI / 180) * Math.sin(latB * Math.PI / 180);
        var s = x + y;
        if (s > 1)
            s = 1;
        if (s < -1)
            s = -1;
        var alpha = Math.acos(s);
        var distance = alpha * earthR;
        return distance;
    },
    outOfChina: function(lat, lon) {
        if (lon < 72.004 || lon > 137.8347)
            return true;
        if (lat < 0.8293 || lat > 55.8271)
            return true;
        return false;
    },
    transformLat: function(x, y) {
        var ret = -100.0 + 2.0 * x + 3.0 * y + 0.2 * y * y + 0.1 * x * y + 0.2 * Math.sqrt(Math.abs(x));
        ret += (20.0 * Math.sin(6.0 * x * this.PI) + 20.0 * Math.sin(2.0 * x * this.PI)) * 2.0 / 3.0;
        ret += (20.0 * Math.sin(y * this.PI) + 40.0 * Math.sin(y / 3.0 * this.PI)) * 2.0 / 3.0;
        ret += (160.0 * Math.sin(y / 12.0 * this.PI) + 320 * Math.sin(y * this.PI / 30.0)) * 2.0 / 3.0;
        return ret;
    },
    transformLon: function(x, y) {
        var ret = 300.0 + x + 2.0 * y + 0.1 * x * x + 0.1 * x * y + 0.1 * Math.sqrt(Math.abs(x));
        ret += (20.0 * Math.sin(6.0 * x * this.PI) + 20.0 * Math.sin(2.0 * x * this.PI)) * 2.0 / 3.0;
        ret += (20.0 * Math.sin(x * this.PI) + 40.0 * Math.sin(x / 3.0 * this.PI)) * 2.0 / 3.0;
        ret += (150.0 * Math.sin(x / 12.0 * this.PI) + 300.0 * Math.sin(x / 30.0 * this.PI)) * 2.0 / 3.0;
        return ret;
    }
};