The direction in which the boat actually moves over the seabed, measured by GPS.
COG (course over ground) is the direction the boat actually travels over the seabed, measured by GPS in degrees from north. It's where you're really going, which is not necessarily where the bow is pointing.
COG and heading
The heading is the direction the bow points. Two things pull COG away from it:
- Leeway: the wind pushes the boat sideways through the water.
- Current: the water itself moves and carries the boat with it (see set and drift).
Under engine in still water COG and heading are almost the same. Close-hauled in a cross current they can differ by 10° or more.
In practice
- On the chartplotter, the COG line ahead of the boat shows where you'll end up if nothing changes, useful for checking you clear a buoy or headland.
- COG is usually shown relative to true north; most devices can switch to magnetic (see magnetic variation).
- At very low speeds GPS can't tell the direction reliably, so COG wanders when you're at anchor or drifting.