Sounding, tidal height and space below the keel

A charted depth refers to a declared vertical plane, the chart datum. Tidal height describes the predicted water surface relative to a reference.

When both use the same datum, adding height and depth gives static water depth in the model. Mixing a height relative to mean sea level with a sounding relative to chart datum can produce correct arithmetic with incompatible references.

The Portuguese Hydrographic Institute describes Portuguese ZH as a reference below the lowest astronomical tide. Its relationship to mean sea level depends on location.

Other services and charts may use different references. “Chart datum” must not be interpreted as one universal plane.

Positive soundings place the bottom below the reference; drying areas may indicate height above it.

An astronomical prediction does not automatically include every meteorological effect. Observed level can differ through wind, pressure and local factors.

Nor can local current be obtained from tidal height alone: horizontal movement has its own dynamics. A smooth height curve does not by itself supply an entrance current or slack-water time.

This lesson identifies components and references of a static calculation. It gives no minimum clearance, passage window or interpolation rule.

Those decisions require current local sources, knowledge of the individual vessel and specialist assessment. Tidal oceanography is developed separately without replacing navigation data.

Waterline, lowest vessel point and chart zero are distinct references. An acoustic depth value can include a configured transducer offset.

If it already represents distance below the keel, subtracting draught again duplicates a component. The diagram separates vertical relationships to explain that problem without selecting an installation offset.

System and vessel documentation establish actual context.

Surface height varies through time. Two heights of 1.0 and 2.0 m do not establish a midway height of 1.5 m: development may be nonlinear.

A tide curve has its own form and origin. Arithmetic averaging is no universal interpolation rule.

This lesson therefore demonstrates addition of simultaneous compatible values, without interpolating local tables or choosing an application method.