code: public static double getResolution( int zoom, int tileSize ) {
          // return (2 * Math.PI * 6378137) / (this.tileSize * 2**zoom)
          double initialResolution = 2 * Math.PI * 6378137 / tileSize;
          return initialResolution / Math.pow(2, zoom);
      }
docs: Resolution (meters/pixel) for given zoom level (measured at Equator)
  
  @param zoom     zoomlevel.
  @param tileSize tile size.
  @return resolution.