Hydrographic Survey Notes

Chart Datum

Diagram of Tide Levels

Depths in the Thames throughout the Port of London are measured to chart datum, which is defined as a level so low that the tide will not frequently fall below it. This level approximates to Lowest Astronomical Tide (L.A.T.) which is the lowest level that can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions.

(For definitions click here).

Chart Datum is also the level to which tidal levels and predictions are measured and is the same as the zero of the tidal predictions.

In the River Thames we use a step datum as the slope of the river makes it difficult to use L.A.T. - i.e. the gradient along the river is changing rapidly and therefore we assign a specific chart datum value for small sections along the river. These steps are spread out and the difference between each is fairly small, usually 10-15 cm, and are on average 2-2.5km apart.

Click on the image for a sample pdf (21kb).

Connection between Chart Datum and Land Survey Datum

The chart datum steps on the Thames are referenced to the UK vertical land datum called ODN (Ordnance Datum Newlyn). This refers all chart datum levels to a common, national horizontal plane and allows the levels to be checked. Such datums are referenced to fixed points called bench marks. The Ordnance Survey (OS) is Britain’s National Mapping Agency and maintains the national vertical datum.

A chart datum value is dependant upon the range of the tide at a particular location. It varies from place to place and therefore is not a suitable national reference plane.

Tide levels on the Thames

The PLA provides tables of tide predictions and actual, real-time tidal levels at various tide stations along the river and out to the estuary. The levels of both the predicted tide and the actual tide are measured from chart datum.

As the River Thames has a semi-diurnal tidal regime, i.e. a pattern of 2 high waters and 2 low waters every day, the predictions and actual observations will show two maximum heights and two minimum heights every 24 hours.

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Charting on the Thames

All depths on a chart are measured below chart datum. If the tide level were to fall to chart datum, i.e. zero, then the depth shown on the chart would be the actual depth at that time in the river. Likewise, if you know the charted depth at a particular location and you know the actual, real-time tide level then you will know the total depth of water at that time, i.e. charted depth + tide level = total depth.

Negative depths or underlined depths (2₃) are depths above chart datum (also known as drying heights) and represent the area on each side of the river - the beaches or mud-flats - or a drying bank in the estuary that will be exposed or dry-out at low water.

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Click on the link for more information on the Hydrographic Service.