The tide is a prediction. The water has the last word.

Why the number at a tide station can differ from the curve on your screen.

A tide prediction describes the familiar rise and fall associated with the gravitational influence of the Moon and Sun. A water-level observation is what a station actually measured. They are related, but they are not interchangeable.

Wind, atmospheric pressure and other local conditions can shift observed water levels away from the predicted tide. On Wakewing, the current water-level card shows an observation, while the curve shows predictions. Each carries its own time and source.

Every height also needs a reference. We display station water levels relative to Mean Lower Low Water, abbreviated MLLW. This is a local tidal datum, not a universal elevation and not the depth of water beneath a boat. Heights at different stations should not be treated as points on one identical reference surface.

Use the curve to notice the rhythm of a harbor. Check the observation time to understand the freshness of a measurement. For operational decisions, consult official forecasts, charts and appropriate instruments. A calm-looking curve does not describe every condition on the water.

Read more: NOAA tidal datums ↗