MapBox Static API Positioning

<!DOCTYPE html>
<html>
    <head>
        <title></title>
    </head>
    <body>
        <img id='i' /><img id='i2' />
        <script>

// Closures including constants and other precalculated values.
        var cache = {},
            EPSLN = 1.0e-10,
            D2R = Math.PI / 180,
            R2D = 180 / Math.PI,
            // 900913 properties.
            A = 6378137,
            MAXEXTENT = 20037508.34;
        // SphericalMercator constructor: precaches calculations
        // for fast tile lookups.
        function SphericalMercator(options) {
            options = options || {};
            this.size = options.size || 256;
            if (!cache[this.size]) {
                var size = this.size;
                var c = cache[this.size] = {};
                c.Bc = [];
                c.Cc = [];
                c.zc = [];
                c.Ac = [];
                for (var d = 0; d < 30; d++) {
                    c.Bc.push(size / 360);
                    c.Cc.push(size / (2 * Math.PI));
                    c.zc.push(size / 2);
                    c.Ac.push(size);
                    size *= 2;
                }
            }
            this.Bc = cache[this.size].Bc;
            this.Cc = cache[this.size].Cc;
            this.zc = cache[this.size].zc;
            this.Ac = cache[this.size].Ac;
        };

// Convert lon lat to screen pixel value
        //
        // - `ll` {Array} `[lon, lat]` array of geographic coordinates.
        // - `zoom` {Number} zoom level.
        SphericalMercator.prototype.px = function(ll, zoom) {
            var d = this.zc[zoom];
            var f = Math.min(Math.max(Math.sin(D2R * ll[1]), -0.9999), 0.9999);
            var x = Math.round(d + ll[0] * this.Bc[zoom]);
            var y = Math.round(d + 0.5 * Math.log((1 + f) / (1 - f)) * (-this.Cc[zoom]));
            (x > this.Ac[zoom]) && (x = this.Ac[zoom]);
            (y > this.Ac[zoom]) && (y = this.Ac[zoom]);
            //(x < 0) && (x = 0);
            //(y < 0) && (y = 0);
            return [x, y];
        };

// Convert screen pixel value to lon lat
        //
        // - `px` {Array} `[x, y]` array of geographic coordinates.
        // - `zoom` {Number} zoom level.
        SphericalMercator.prototype.ll = function(px, zoom) {
            var g = (px[1] - this.zc[zoom]) / (-this.Cc[zoom]);
            var lon = (px[0] - this.zc[zoom]) / this.Bc[zoom];
            var lat = R2D * (2 * Math.atan(Math.exp(g)) - 0.5 * Math.PI);
            return [lon, lat];
        };

var sph = new SphericalMercator();
        var i = document.getElementById('i');
        var i2 = document.getElementById('i2');
        var loc = [-77.04,38.89];
        var s = 640;
        i.src = 'https://api.tiles.mapbox.com/v3/mapbox.mapbox-light/' + loc + ',13/640x640.png';
        var px = sph.px(loc, 13);
        px[0] += s;
        var loc2 = sph.ll(px, 13);
        i2.src = 'https://api.tiles.mapbox.com/v3/mapbox.mapbox-light/' + loc2 + ',13/640x640.png';
        </script>
    </body>
</html>