Tides: heights, streams and timing

Charted depths and drying heights are referenced to the datum named on the chart. A predicted height of tide is added to a charted depth to estimate total water depth at a time and place. Drying heights need their own sign-aware treatment; they are not ordinary soundings.

The chart, tide table and secondary-port method must be compatible. Mixing a height from one reference port with an unrelated curve or using the wrong day can produce precise-looking but invalid working.

A tidal stream has a direction, called set, and a rate, called drift. It changes the vessel's track over the ground and the time needed to reach a waypoint. The relevant stream is the one expected during the passage interval, not simply the value printed nearest the departure point.

Predictions describe expected astronomical tide, while pressure, wind and local effects can alter observed levels and flow. Draft, squat, heel, wave response, sounding uncertainty and the consequences of touching also matter. Use the method required by the exercise when revising, but use current official data and a deliberately chosen margin for real navigation.