private String tileToQuadKey(final int tx, final int ty, final int zl)
	{
		String quad = "";

		for (int i = zl; i > 0; i--)
		{
			int mask = 1 << (i - 1);
			int cell = 0;

			if ((tx & mask) != 0)
			{
				cell++;
			}

			if ((ty & mask) != 0)
			{
				cell += 2;
			}

			quad += cell;
		}

		return quad;
	}