Tides

Question 1

When to correct for British summer time?
BST (UT +1)

Answer 1

Last Sunday of March -
Last Sunday October

Question 2

With the aid of a sketch, explain Chart Datum, LAT, MHWS, MHWN, MLWN, MLWS, HOT,

Answer 2

Chart Datum is an arbitrarily fixed level, usually below low tide level. The Admiralty standard is to use the level of LAT (see below). All tidal heights are measured above Chart Datum and all depths shown on charts are measured below chart datum. Some charts, based on non-UK data, may use a level other than LAT as chart datum. In such cases tides may occasionally fall below
Chart Datum and will appear in the tables as negative tides.

Under Keel Clearance (UKC) is the depth of water, usually in metres, between the vessel’s bottom and the seabed or obstruction. It is equal to the actual depth of water - draft of vessel.

Height of Tide (HOT) is the height of the water level above chart datum. The height of tide is constantly changing and its value can be calculated for any given instant by using tide tables. The actual depth of water at any time is thus the sum of charted depth + height of tide.

Drying Height is the height of an object such as an obstruction, rock or shoal above chart datum.
Depending on the height of tide, this obstruction, rock or shoal may or may not be visible above the surface

MHWS stands for Mean High Water Springs. On Admiralty charts heights of lighthouses and bridges and other land features (e.g. spot heights) are measured above this level.

LAT stands for Lowest Astronomical Tide and is the lowest tide predicted in a 19 year cycle.