Interesting aside.

Have a look at the little chartlet on page 16 of the Anglesey Pilot, which I believe was taken from Capt Goddard's chart. Notice the 4 ft sounding over Cheese Rock. Every other sounding shown on this chartlet can also be immediately identified with Goddard's chart. But the chartlet doesn't state the datum against which these soundings are reduced. Since we have Capt Goddard's chart, we happen to know that these soundings are reduced to below a level of 11.25 feet below Mean Tide Level. But without that little bit of information the sounding figures shown on this widely published little chartlet are completely meaningless.