@Override
	public String getTileUrl(int x, int y, int zoom)
	{
		int ty = y;
		int tx = x;

		// int width_in_tiles = (int)Math.pow(2,pyramid_top-zoom);
		int width_in_tiles = getMapWidthInTilesAtZoom(zoom);
		// System.out.println("width in tiles = " + width_in_tiles + " x = " + tx + " y = " + ty);
		if (ty < 0)
		{
			return null;
		}
		if (zoom < midpoint)
		{
			if (ty >= width_in_tiles / 2)
			{
				return null;
			}
		}
		else
		{
			if (ty != 0)
			{
				return null;
			}
		}

		String url = this.baseURL + "/" + zoom + "/" + ty + "/" + tx + ".jpg";
		// System.out.println("returning: " + url);
		return url;
	}