What datum are tide heights measured against?

Tide heights are measured against a defined vertical reference level called a tidal datum. The datum provides a consistent reference point from which predicted tide heights can be calculated and displayed.

The particular datum used can vary depending on the tidal station and the source of the tidal data. This means that tide heights should always be considered in the context of the reference datum used for that location.

This is also one reason why tide heights from two different sources should not necessarily be compared without checking that they are using the same reference level.