Cross-Track Error Tolerance: Set Autopilot Limits

What cross-track error is

Cross-track error is the distance between a boat and the track line it is supposed to follow. When an autopilot runs in route-following mode, it does not simply hold a compass heading. It uses position data from the network and steers toward the next waypoint, trying to keep the boat on the line between waypoints.

An autopilot that only holds a heading keeps the bow pointed in one direction. The boat may drift off that line because of wind, current, or a steering system that does not hold a perfectly straight course. Route-following mode adds a position check: the autopilot compares where the boat is with where the track says it should be. The difference between those two positions is the cross-track error.

• Route following is the mode that uses this error and steers to reduce it.
• Heading mode holds a compass direction and does not use cross-track error.
• The tolerance setting decides how much error is allowed before the autopilot acts.

The tolerance is a threshold, not a precision rating

The tolerance setting tells the autopilot how far off the track line it may go before it reacts. It is a threshold, not a promise about course accuracy. A narrow threshold means the autopilot corrects as soon as the boat has drifted a short distance; a wide threshold means the autopilot waits until the boat has drifted farther.

The same idea is sometimes described as a deadband: a zone around the track in which no correction happens. The wider the deadband, the more the boat may wander before a correction is triggered. The narrower the deadband, the sooner the system acts. The deadband setting is closely related and is often adjusted in the same menu.

Because the tolerance is a setting, you can change it. What values are available, and what the autopilot does with them, is determined by the control head menus and the maker's manual, not by the listing you are reading.

When a tight limit helps

A tight tolerance suits times when you want the boat near the line: open water, an area with little traffic, or a long straight leg where the track is well clear of obstructions. The autopilot reacts quickly, so the boat spends less time off the line.

Expect the autopilot to make more corrections with a tight tolerance, because the threshold is crossed more often. That is a consequence of the setting, not a sign that the system is working poorly.

This is a place where the control head matters. A head with clear menus makes the tolerance easy to find and change. Buyers comparing autopilots with a control head should look for a head that exposes navigation settings, or a pack that includes one.

When a wide limit helps

A wide tolerance gives the autopilot a bigger band to work in. The boat is allowed to wander farther before a correction starts. That can be the right choice when you would rather have fewer helm movements than a perfectly straight track.

Situations that suit a wide tolerance include open water with plenty of sea room, a low-speed leg where the track line is not critical, or a stretch where you do not want the helm busy. The setting is a way of telling the autopilot how much effort to spend on the line.

As with a tight tolerance, the values available come from the autopilot's menus. The maker's manual lists what the range is, and a qualified installer can help interpret it for your boat.

Where the setting lives and what it is called

The tolerance is not part of the drive unit. It is a navigation setting, so it lives in the autopilot's software and in the menus of a control head or a connected display. The name varies between makers: cross-track limit, track error limit, off-track limit, or a deadband term.

On many systems the setting only matters in route-following mode. In heading mode, the autopilot steers a compass course and cross-track error is not used. The setting is separate from an off-course alert: the alert tells you when the boat has deviated beyond a chosen limit, while the tolerance drives the correction.

The manual is the source for the menu path and for whether the setting can be changed while underway. If the manual is not clear, ask the maker or a dealer before you buy.

The data behind route-following

Route-following needs data. The autopilot must know where the boat is and where the track goes. That data travels over a marine network. NMEA 2000 is a CAN-based network standard that interconnects marine electronics and lets compatible devices share data, including commands and status. Position and route information are part of that shared data.

An autopilot that can follow a route receives a waypoint list from a chartplotter or another navigation source. The cross-track error is computed from that route data and from the boat's position. Without a position source and a route, the autopilot works in heading mode, and the tolerance setting is not used.

Autopilots also manage how they approach waypoints. For example, Furuno describes Route Smoothing on the NavPilot 300: the autopilot decreases the speed of turns as the boat approaches waypoints, because a fast approach can overshoot the waypoint and cause an undesirable deviation. The tolerance and the turn behavior are different settings, but both shape how closely the boat follows the route.

What to ask before you buy

The tolerance is a software setting, so the buying question is about the control head and the menus, not about the drive in the box. Before you order a pack, ask these questions.

• How is the tolerance labeled on the control head, and is it adjustable in route mode?
• Does the setting apply to route-following only, or also to other modes?
• What does the manual say about changing the tolerance while underway?
• Is the off-course alert tied to the same value or set separately?
• Does the pack include a control head that exposes navigation settings, or is the head sold separately?

What this means for the pack you buy

A buyer who plans to follow routes should look at complete autopilot packs because they show what is in the box. The tolerance itself is not a boxed part; what matters is that the control head and the software expose it.

The watch stays with you

An autopilot is a steering aid. It can hold a heading or follow a route, but it does not replace a lookout. The operator remains responsible for watching traffic, hazards, and the boat itself.

The United States Coast Guard Boating Safety Division is dedicated to reducing loss of life, injuries, and property damage on U.S. waterways by improving the knowledge, skill, and abilities of recreational boaters. The autopilot takes over the helm, not the watch.

What to pick for your situation

Questions

What is cross-track error?

Cross-track error is the distance between the boat and the intended track line between waypoints. The autopilot computes it in route-following mode from position data and the route.

Does a tighter tolerance mean more accurate steering?

No. The tolerance is the threshold that triggers a correction. A tighter threshold makes the autopilot react sooner; it is not a rating of how precisely the system holds a course.

Can I change the tolerance while underway?

That depends on the control head and the maker's manual. Some systems let you change navigation settings while underway; others do not.

Does the tolerance work when I am not following a route?

No. In heading mode the autopilot holds a compass heading and cross-track error is not used. The tolerance applies in route-following mode.

Is the tolerance the same as an off-course alarm?

They are related but separate. The tolerance drives the correction; the off-course alert tells you when the boat has deviated beyond a chosen limit.

Who sets the tolerance?

The owner or a qualified installer, following the maker's manual. The manual lists the menu path and the values the system accepts.