code: function (coord, zoom, ownerDocument) {
        var key = zoom + '/' + coord.x + '/' + coord.y;
        if (!this.cache[key]) {
          var img = this.cache[key] = new Image(256, 256);
          this.cache[key].src = this._getTileUrl(coord, zoom);
          this.cache[key].setAttribute('gTileKey', key);
          this.cache[key].onerror = function () { img.style.display = 'none'; };
        }
        return this.cache[key];
      }
docs: Get a tile element from a coordinate, zoom level, and an ownerDocument.