Waypoint
A Waypoint is a fixed geographic point, defined by its latitude and longitude, used to build up a planned route. A passage is planned as a series of straight legs joining a sequence of waypoints, at each of which the ship typically alters course. Waypoints are entered into the ECDIS and, for track-control steering, into the autopilot.
Good waypoint placement keeps the route clear of dangers, allows for the turning circle of the ship at each alteration, and marks points where the navigator checks progress and cross-track error. The route through the waypoints is the backbone of the voyage plan.
Waypoints turn a voyage plan into something the ship can actually steer and monitor. Placing them to keep safe clearances and account for the turning circle is basic passage-planning safety; a badly placed waypoint can put the ship into danger at an alteration of course.
What is cross-track error?
The distance the ship has strayed to one side of the planned track between two waypoints. The ECDIS and autopilot monitor it so the navigator can correct back onto the intended route.