Hon. Dir. of Pub. Works,
C.O.
42137
14 DEC 04)
332
Adverting to C. O. D. 273 of 1908, I have to point out that the latest Admiralty Chart to hand, viz., No. 3429, of 1904, West Lamma Channel, shows an apparent improvement in the depth of the water off the North-west corner of Lamma Island, at the point to which I drew attention in paragraph 41 of my report of 30th July, 1904.
Reference to the title of the chart, however, shows that a new zero has been adopted for the soundings. This zero is 4.03 feet below mean sea level, or about 1.5" higher than the zero used by the "Rambler" during the last survey of the Harbour; and a glance at Chart No. 1466 of 1902 shows that the older survey was imperfect.
The change of zero, and the perfecting of the survey, is sufficient to account for the alteration in the contours. It is not likely that any scouring has taken place.
I have shown by red lines on the new chart, the positions occupied by the 3 fathom contours on the old chart (No. 1466 of 1902).
I have compared the contours at the mouth of Sulphur Channel, as shown by Charts 3280 and 3429, and find that they occupy exactly the same positions in both charts, although they ought to differ on account of the difference of zero.
12th November, 1904.
(84.) J. F. Boulton.