SOG (Speed Over Ground) is a sailing term representing a vessel's actual speed relative to the Earth's surface (seabed), measured by GPS. It differs from speed through water (STW) by incorporating the effects of currents, tides, and wind. SOG tells you how fast you are actually moving toward your destination.

Key Aspects of SOG:
Definition: The true speed of the vessel compared to the ground, often termed "speed over the chart".

Measurement: Primarily measured via GNSS/GPS, providing a direct measurement of position change over time.

Impact of Current: If you are sailing with a current, SOG will be higher than your speed through the water (STW). If sailing against a current, SOG will be lower than STW.

Usage: SOG is critical for navigating, estimating arrival times, and optimizing routing, but it does not measure how fast the boat moves through the water (essential for trim).

Related Term: COG (Course Over Ground) works with SOG to show the true, actual path taken over the ground.