A tide datum is the reference level used to measure tide height. The same tide can have a different reported height when measured from a different datum, so it is important to use matching datums when comparing Tide Guide with a chart, harbor guide, or another forecast.
To change the datum, go to Settings > General > Tide Datum and choose the supported reference level you want to use. Tide Guide refreshes the station data after you make a selection.
Available datums depend on the selected station and its data provider. NOAA-backed stations use Mean Lower Low Water (MLLW). When comparing or sharing tide heights, make sure everyone is using the same datum.