Cruising

Avoid a GPS-induced incident

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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.

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