Navigation and Seamanship (1+1)

1.4.5.3. Chat datum

Chart datum: This is a level of the water usually considered to be the depth at the lowest astronomical tide (Chart datum being an imaginary datum, beyond which the sea level rarely falls). This is a depth below which the level rarely falls, though it may not always be the case. All depths shown on a chart are below this level and these depths are called “soundings”. The unit of the soundings is as per the legend under the title. This datum is also used on the charts for giving drying heights of features, which are periodically covered and uncovered, by the tide.