Marine Glossary

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cross track error

When sailing a route, we are following a course line between sets of waypoints (WP). Going from WP2 to WP3, for example, WP3 will be the active target, and the line on the chart between WP2 to WP3 is the track we intend to follow. As we proceed, wind, current, helming errors, or various other errors might lead us to drift off of this intended track line. One way to detect this is to display the perpendicular distance from the track line to us at any moment. Our ECS program can compute this automatically. This is called the cross-track error, abbreviated XTE. It is usually expressed in yard or meters, maybe switching to fractions of a nautical mile at large errors. It will also be labeled in such a way that we know if we are to the right or left of the track. How this is done varies with every nav program. We can also zoom into our track on the chart to see clearly where we are relative to where we want to be.

Once we detect a notable XTE, our job is to get back onto the track as best we can, and if we cannot, then we must look ahead to the consequences of this limitation.

Many programs allow us to just make a mark on the chart and then click "navigate to this mark." Usually this is not a good way to navigate, because even if you know at all times the range and bearing to your target, this type of navigation does not create a reference track, so you can slip well off of the initial heading to the mark that was true when you activated it. In short, there is no XTE as a guide. Other programs, such as qtVlm, will plant a mark at your present position at the moment you choose a single mark to go to, and this way you then have a permanent track as a record and XTE will work. It is valuable to understand how this one point navigation works on your ECS.

Usually monitoring the Heading Line on our vessel icon compared to the COG predictor line also extending from our vessel icon is the fastest way to detect that we might be slipping off of our intended track. (See an example in action.)

Abbreviation: XTE