GPS has greatly simplified certain aspects of navigation; at the mere touch of a button, a boat’s position can be determined within about 30 feet. Despite the reliability of these devices, boats are still being damaged because of navigational errors. After several decades of both navigating and teaching navigation, I’ve suffered a few close calls of my own and can recommend some ways to use your GPS responsibly.
• Unless you are racing, don’t set a waypoint directly on a lighthouse, a buoy, or any other fixed aid to navigation. GPS is so accurate that if you run an entire leg to the waypoint with a tired crew or in poor visibility, there’s a chance you may hit the object. In one well-known incident, a delivery crew aboard a large sailboat first set a waypoint on a lighthouse off the Maine coast and then set the autopilot to steer to the waypoint. Both the GPS and the autopilot worked perfectly, and the boat ran hard aground on the rocks surrounding the light. It is best to set waypoints near—but a safe distance away from—physical objects.
• Don’t confuse a GPS unit’s accuracy range with its arrival-alarm radius. Most GPS units are accurate to within about 30 to 50 feet, and WAAS- or DGPS-enabled units may be accurate to within 10 to 15 feet. The waypoint arrival-alarm radius is an entirely different matter; the arrival alarm sounds when the boat is within some preset distance or time-to-go from the waypoint. A typical value is 0.1 nautical mile, but this can differ between GPS units and, on some units, can be adjusted. Figure 1 shows how misinterpreting an alarm radius can lead to an accident. In order to avoid problems, always factor in the alarm radius setting when you are establishing your waypoints.
• Make sure you are on the right page and are reading the right numbers. The multiple display pages on a GPS unit usually include your present position, position of the waypoint, and so forth. Last summer, while sailing in the Gulf of Maine, we were surrounded by fishing boats and, just as the fog rolled in, the radar quit. We broadcast a scurit call to alert the fishing fleet to our position—and were promptly informed by the skipper of one of the vessels that our position made no sense. When we checked it, we saw that he was absolutely right. We had broadcast the position of our next waypoint, not our current position. Preventing this kind of error is as simple as double-checking the data.
• Make sure that your GPS is displaying values in the appropriate units. A GPS display can be set to show statute miles or nautical miles, true or magnetic bearings and courses, and so forth. This isn’t generally an issue when using a permanently installed unit, but if, for example, you take your handheld on a hike and forget to reset it to nautical miles, the distances will not be correct—unless you happen to be on the Great Lakes.
Subtler, but equally important, is the unit’s position display: it reads either in degrees, minutes, and seconds or in degrees, minutes, and decimal minutes. Use the former with a large-scale chart and the latter with small-scale charts. Obviously, you must know how to change the display when the situation requires it.
• Understand what a GPS display is really telling you. For example, a GPS unit can measure a vessel’s speed, but the speed shown is over the ground and not through the water. If you sail where there are significant currents, the difference between the two can be substantial. To determine the speed of the current and factor it into your navigation, you must also have a conventional knotmeter.
• Become proficient with your GPS before you need to rely on it. Although most units are reasonably easy to operate, certain functions, such as establishing waypoints and routes, can be tricky if you are not familiar with them. Leafing through the operator’s manual for the first time when conditions are challenging is not good seamanship. Raise your competency level when you really have time to study and learn—when conditions are favorable or when you are ashore.
• A GPS position may be more accurate than the chart on which it is plotted, particularly if the chart was created with sextants, triangulation, and other “manual” devices. The position of an object on these charts can be substantially different—1,000 yards or more—from the GPS position, and the consequences of such a difference can be significant (Fig. 2.)
This point was driven home on a recent cruise I made in the Caribbean when I discovered that the GPS coordinates and those on the (old) chart I was using differed from each other by more than 500 yards. Fortunately, I quickly noticed the discrepancy. If I hadn’t, I could have been in serious trouble when navigating through, say, a narrow channel.
Although charts are constantly being updated, especially those used in popular sailing areas, you should always check the chart’s accuracy against the GPS with visual bearings on fixed objects. Also note the datum printed on the chart. If it says NAD 83 or WGS 84, it is relatively new and you are less likely to have problems.
• Carry a backup GPS unit, because all electronics will eventually fail. I like to carry a spare that is similar or identical to the main unit; if I should have to activate the spare, there’s no learning curve involved. Also be aware that most units go through a cold start the first time you turn them on in a new area; this means it may take several minutes, or longer, to establish a fix while the unit is searching for the satellites. That’s why, if you are passing through navigationally tricky waters, it’s a good idea to start up the spare beforehand; once the spare unit has a fix, you can turn it off. Periodically turning on the spare also allows you to check that its batteries are strong and functional.
• Know how to navigate without having to rely on GPS. Because a GPS unit continually displays your position and other relevant navigational data, it’s easy to get overconfident. However, any unit might stop working. One friend of mine was on a delivery last year when suddenly the GPS no longer displayed the boat’s position. No problem, he thought, and he turned on a spare unit. But it wouldn’t display a position either. Two additional units also failed to give a position. The situation could have been serious because he was approaching the entrance to Chesapeake Bay in near-gale-force conditions. Fortunately, he is a proficient navigator and had been keeping a log; he was able to use dead-reckoning to plot and track his position on a chart, and he made a safe landfall. Twelve hours later, all the GPS units began working again.
Although he never got a definitive explanation, it’s likely he passed through a military exercise; GPS units are very easy to jam.
• Never rely exclusively on GPS. Learn piloting and dead-reckoning techniques and maintain your proficiency through practice. You can also use these techniques to double-check your electronics. During a delivery from Maryland to Newport, Rhode Island, we were sailing offshore near Cape May, New Jersey.
It’s a tough stretch of water, and full of shoals. Although the boat had a differential GPS and a state-of-the-art chartplotter, we used four different data sources simultaneously: the GPS/chartplotter, the depthsounder, radar, and visual observations that included watching for breaking waves. We had a paper chart laid out and plotted our position on it frequently. Overkill? Perhaps. But if the GPS had somehow malfunctioned, we would have known immediately. And, more important, we would have known exactly where we were.
It’s true that GPS has revolutionized navigation; when you use it, you know exactly where you are. But this has created another navigational challenge: You need to use GPS properly, as well as traditional piloting and dead-reckoning techniques, to get you where you want to go both safely and efficiently.
Larchmont, New York–based Steven Henkind, M.D., Ph.D. is a marine safety consultant who has served as captain, navigator, and crew on sailing vessels up to 295 feet. He also holds a 200-ton U.S. Coast Guard license.