Speed Over Ground (SOG)

The actual velocity of a vessel relative to the Earth’s surface, accounting for the effects of currents, tides, and other environmental factors. 

Speed Over Ground (SOG) is a critical navigational term in maritime operations, representing the true speed at which a vessel moves over the Earth’s surface. Unlike Speed Through Water (STW), which measures the vessel’s speed relative to the water it is moving through, SOG takes into account external influences such as tidal streams, ocean currents, and wind-driven drift. This distinction is vital for accurate navigation, as a vessel’s progress towards its destination is determined by its SOG rather than its STW. 

SOG is typically measured using GPS (Global Positioning System) technology, which calculates the vessel’s movement relative to fixed points on the Earth’s surface. This measurement is displayed in knots (nautical miles per hour) and is crucial for route planning, fuel efficiency calculations, and ensuring timely arrivals. For example, if a vessel is travelling at 10 knots through the water but is being pushed back by a 2-knot opposing current, its SOG would be 8 knots. Conversely, if the current is aiding the vessel, the SOG could exceed the STW. 

Understanding SOG is essential for safe and efficient navigation, particularly in coastal waters where tidal streams can significantly impact a vessel’s progress. Mariners must constantly monitor and adjust for these factors to maintain their intended course and schedule. By integrating SOG with other navigational data, such as heading and position, sailors can optimise their voyage and avoid potential hazards.