When setting a course on a GPS, what indicates if you are getting off the prescribed course?

When setting a course on a GPS, cross-track error is the term that indicates how far off the vessel is from the intended path. It measures the shortest distance from the vessel's current position to the projected course line. A larger cross-track error means that the vessel is further from its designated course, prompting the navigator to take corrective action.

In contrast, terms like deviation, waypoint error, and track error do not accurately reflect the specific measurement of how off-course a vessel is in relation to a pre-defined path. Deviation often refers to errors caused by magnetic interference affecting compasses, while waypoint error relates to accuracy in reaching specific waypoints along a course. Track error might indicate the difference between the intended track and the actual ground track over time, but it does not pinpoint the immediate deviation from a set path like cross-track error does. Thus, cross-track error is the most relevant measure for assessing how far off a prescribed GPS course a vessel has strayed.