Cross-Track Error (XTE or XTK) is the perpendicular distance a vessel is to the left or right of its planned, straight-line course between two waypoints. It helps sailors immediately understand if they have drifted due to wind or current and shows how far to correct to return to the intended track.

Key Aspects of XTE:
Measurement: Usually displayed in nautical miles, yards, or meters on chartplotters, GPS units, and navigation apps.

Purpose: Essential for maintaining safe navigation within channels or avoiding hazards identified in the passage plan.

Autopilot Interaction: When following a route, the autopilot uses XTE data to keep the vessel on the intended line (cross-track error zero).

Limitations: XTE is less effective when sailing upwind and tacking across a designated course, as the vessel intentionally moves away from the straight-line track.

An XTE alarm is typically set to alert the helmsman when the boat strays too far from the planned track.