Tide heights, chart datum and mAOD

Tide times are easy to compare. Tide heights need one extra piece of context: the zero level they are measured from.

What mAOD means

mAOD means metres above Ordnance Datum. It is a land-surveying reference level used by the Environment Agency gauges. A tide height of 1.5 mAOD means the sea surface at the gauge is predicted to be 1.5 metres above that datum.

Why printed tide tables look different

Many harbour tide tables use chart datum, a local nautical datum chosen so charted depths are usually positive. Ordnance Datum and chart datum are not the same zero. That is why a height on Tide Checking may not match a printed tide-table height for the same place and time.

How to use the numbers safely

Use heights to compare tides at the same gauge: a 5.0 m range is bigger than a 3.2 m range, and a higher high water will reach farther up the shore. Do not use the height as a depth of water at a mooring, channel or harbour entrance.

Why we keep the gauge datum

The site is built from Environment Agency observations. We keep the gauge datum visible because guessing local chart-datum offsets would create false precision. Where a place does not have a validated gauge, the site says so instead of shifting the numbers.