How do I calculate the depth of water using soundings and tide heights?
A sounding on a chart shows the depth of water at a 0.0 m tide (at LAT datum), but the tide rarely falls to 0.0 m. The tide values seen in Maritime Safety Queensland’s Official Tide Tables and Boating Safety Guide (and in newspapers, weather reports, and so on) are also calculated to LAT datum. Therefore, to determine how much water is at a certain point on a certain day, add the sounding on the chart to the tide value for that day, time and locality. For example, if the sounding is 2.3 m and the tide is 1.8 m at the Brisbane Bar at the time of high tide, the total water available near that sounding is 4.1 m. The exception to this rule is ‘dry soundings’. Because dry soundings generally represent sandbanks that ‘dry’ as the tide falls, a dry sounding has to be subtracted from the tide value to get the water depth at that point. For example, if your course took you over a dry sounding of 0.4 m, and the tide was 1.8 m at the time of high tide, the actual water available at that dry sound
