Waterproof Charts 38G North Bahama

hi my name is Jim cook and I'm one of the instructors for our learn to sail program and this module in our coastal navigation series is titled chart symbols we will start with depths all depths are relative to data datum is the average low water over 18 a little bit more than 18 18 and a half years now why eighteen and a half years it seems like a really weird number the solar year is 18 and 1/2 earth years long so in other words the position of the Stars the Sun the Moon the planets to each other I'll repeat themselves about once every 18 and 1/2 earth years so when we talk about datum we're talking about the average low water in 18 and 1/2 years now there are two datums the United States has their datum and the rest of the world has their datum in the rest of the world they take the lowest water each year for 18 and 1/2 years so in other words the rest of the world averages 18 numbers