Using GPS as a Warning Instrument

Use your chartplotter's features to your full advantage

By Daria Blackwell

Don't let your boat run into the navaid that's between two waypoints. 

In a companion article to the one posted last week, today I am covering the positive aspects of GPS use beyond navigation that many people don't take advantage of.  You have lots of capability in that instrument, and it pays to make full use of it.


Most people don’t realize they can use their GPS receiver as a warning instrument not just a navigation tool.  Often people input two waypoints, one for the start and one for the destination, at times not even checking the route in between to ensure there is no hazard between here and there. We’ve heard of boats arriving at a marina with a navaid stuck in their bows, having passed through their waypoints literally.

You can use your GPS device to help you pass between shoals, rocks and other hazards, identify coastline in fog, and warn you if your anchor drags. Here are some ways to increase your margin of safety while sailing anywhere in the world.

1.      Program multiple waypoints that correspond to check points on coastal passages  

If you have several waypoints, you’ll have the opportunity to check your progress along the way. If you set those waypoints near hazards (navaids, shoals, rocks, etc), it will prompt you to use visual verification of those hazards, making sure that you don’t approach them too closely. Remember that datum can be off on the charts used in chartplotters, sometimes by as much as ¼ mile or more, so the more often you check your position, the safer you will be.

2.      Set proximity waypoints for hazards

Most chart plotters allow you to set a proximity alarm around a hazard that sounds when you enter the circle you’ve set around the hazard. Decide how far a safe distance from the hazard is then set the alarm.  Allow enough of a margin to permit corrective action if the alarm sounds.

3.       Place waypoints on individual hazards

We’ve marked all the rocks (often found the hard way) on the approach to our unmarked inlet. We’ve labelled them Rock underwater, Rock danger, and so on. We’ve also marked the shellfish bed and a shoal to avoid.  That way, we’re adding our local knowledge to the chart on our screen and we can be more easily vigilant each time we pass. You can also set proximity alarms for coastline points in fog, helping you navigate safely along a shore you cannot see.

Most chartplotters also have an anchor alarm. It allows you to ‘drop anchor’ and set an alarm for maximum distance travelled from that point.  You have to allow for drift from the anchor in a full circle or your alarm could be sounding all night long.

4.      Set a cross track error alarm

Your GPS tracks how far your vessel has wandered off track. This can happen as a result of drift in a current, often unnoticed at first because new currents are encountered en route. You can usually set an alarm for maximum cross track error you would allow or for proximity to a hazard.  Find the hazard that is closest to your route and set the cross track error for half the distance you plan to pass it if you maintain course. That way, you’ll be able to correct before getting too near the hazard.

5.      Always use visuals to corroborate what you see on the chart

Check paper charts and look around to make sure you are where your GPS says you are. If you are, proceed with the next leg.  If there’s a discrepancy or you aren’t certain of what you see, stop and take bearings to shore if possible or check other factors like depth contours against data on the chart.  Depth is an excellent independent source of information to verify your position.