Ein Frachtschiff fährt über das offene Wasser.

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Speed over ground and speed through water

A ship has both a speed through the water and a speed over ground. Speed through water (STW) describes movement relative to the surrounding water. Speed over ground (SOG) shows how quickly the ship’s position changes relative to the seabed or a fixed point on land. Both figures describe movement, but use different frames of reference.

Why the figures differ

Currents move the water and therefore affect the ship’s movement over ground. A following current adds to the ship’s speed through the water; an opposing current reduces its speed over ground. If a ship moves through the water at five knots and a following current of two knots is acting, its speed over ground is seven knots, in simplified terms. With an equally strong opposing current, it is three knots. The article Tidal currents and water movement explains the relationship between currents and water levels.

This calculation illustrates movement in the same direction. With a cross-current, the course and speed over ground also change; the movements must then be considered together, by direction and speed. In this case, the difference between STW and SOG cannot be interpreted as the current’s speed alone.

Measurement and display

STW and SOG may come from different sensors. A British navigation guideline describes STW as an input from the water log and SOG as an input from GNSS. The water log measures movement relative to the water; GNSS determines the change in position relative to the Earth. AIS data are based on course and speed over ground; the navigation guideline distinguishes these figures from the through-water values needed for certain collision-avoidance rules. The differences in measurement and display between AIS, radar and nautical charts provide further context.

Both displays can show a speed while referring to different kinds of movement. The STW or SOG label and knowledge of the sensor source help identify what each figure means.

What each figure represents

SOG is the appropriate figure for distance travelled over ground; STW is the one for movement through the water. The unit knot measures speed; the article Nautical miles and knots explains this nautical unit of measurement.

Photo: Markus Lenk