Oceanography

Charts exist because of hydrographic survey. The quality and age of the survey beneath a chart determines how much water the navigator can actually trust.

The shape of the ocean floor and the physical properties of seawater underlie soundings, sound propagation, currents and ice.

Surface currents are wind-driven and organised into gyres; the deep circulation is thermohaline. Both alter ground track, fuel consumption and sea state.

Waves are generated by wind and transform as they enter shallow water. Their period, height and steepness determine ship motion, slamming and the safety of bar crossings.

Ice changes every rule: radar becomes unreliable, charts are sparse, and the margin between routine passage and structural damage is thin.