The Limits of Chartplotters

By Tom Neale

Avoid being led astray by your well-meaning electronics.

Photo of boater checking his chartplotterIt's not enough to know what your plotter is telling you. You've got to understand what it can't tell you as well. (Photo: Billy Black)

The towboat operator took one look and said, "I think we'd better wait." No brainer. The tide was low. The megabucks fast cruiser was in only inches of water, and under those inches was a very hard sand shoal, obvious in the shallow water and very far away from the channel. And somewhere down there were very expensive props and rudders. Floating the boat was out of the question without a blimp the size of the Hindenburg. But the skipper was in a hurry. He had to get this boat to a boat show. "Can't you just tow me off?" The towboat had the capacity to do that, but it would have probably left the bottom in the shoal.

Five hours of waiting and the boat could be pulled off with no additional damage. The haulout revealed twisted running gear and a nightmarish wavy bottom. But the skipper had new props and shafts shipped in, and soon was on his way. "The chartplotter didn't refresh fast enough, so that shoal wasn't showing when I got there" was his explanation. Electrons and a fancy screen don't excuse the navigator from thinking. Here are a few tips for making the most of this remarkable aid to navigation.

Data, Not Intelligence

Chartplotters have made everyday navigation a dream. But, like all technology, they have their limits. The first is the user. As in the case above, if you're going fast enough it's possible that the screen might not refresh quickly enough to keep up with your position. That could be because of processor speed, but it could also be a function of the GPS chip, and the amount of data you're displaying. When you are navigating in areas with obstructions, you need to rely on something lower tech than your chartplotter — your throttle. All you have to do is pull it back. While modern plotters have better processing power and faster refresh times, slowing down when you're unsure can prevent a big headache later.

Refresh rate is just one of several things you need to understand about your plotter. Chartplotters can provide so much information that usually a few hours of training are needed to use them well. But there's a strong temptation to "hook 'em up and start pushing buttons." It's fun and you want to get out on the water. But push those buttons for awhile at the dock with the manual until you become familiar with your new tool. You may be surprised how much information it can provide you, if you just use it correctly.

For example, you need to know if your chartplotter is set to give you latitude and longitude in degrees, minutes, and seconds, or degrees and decimal degrees. Failure to report and/or record position without knowing which "language" is being used has resulted in serious errors, even death.

Getting A Fix

Seeing your boat's icon sailing along on the screen, up in the trees, isn't reassuring. But it's a common event with chartplotters. They're only as accurate as the cartography they're using, and no cartography can be perfect. Things change, particularly on the water. Even the most current and carefully done "maps" can be rendered inaccurate by a storm or grounded barge causing a shoal to shift, or by a change to an aid to navigation. You can and should regularly update your cartography. But even that precaution doesn't completely preclude inaccuracies. Chartplotters can't change the fact that we're boating in the real world. Newer cartography and plotters offer access to "user-generated" data, which is uploaded by your fellow boaters and can in theory show you current bottom contours. But these are still only as accurate as the most recent upload, and you never know whether the person who reported has a properly calibrated depth sounder.

Waypoint adjustment illustrationMany chartplotters can display cross track error (XTE), which tells you the perpendicular distance from your intended course. A large XTE may indicate you're drifting off course. If you're being set to the side in a narrow passage, look at landmarks or aids to navigation both ahead and astern to keep yourself in line with the channel. (Illustration: Marcus Floro)

Even when the cartography is accurate, not all fixes are created equal. The accuracy of your position on the chart depends upon how many GPS signals the chartplotter is receiving, the strength of those signals, and the angle of those signals to one another and to the receiver. Most chartplotters have an icon that will alert you if the accuracy of the fix has deteriorated. If you see that icon, proceed with caution. On most chartplotters, you can check the signal quality by accessing a screen that shows what signals the chartplotter is receiving and how strong they are.

Plotters are only as accurate as the GPS information they're receiving and, as precise as this has become, myriads of things can throw its accuracy off. This includes equipment glitches, from what's on your boat to what's in space, temporary shutdown of satellites for maintenance, and even the possibility of deliberate interference by government entities.

Time And Tide

Chartplotters can do so much that sometimes we forget that they can't do everything. We often use them to establish an ETA for a bridge that only opens on the hour, or to know when we're going to reach an area that will be very shallow at low tide. But traveling on the water often doesn't lend itself well to this sort of prediction.

For example, there are many places on rivers and waterways where the current changes direction as you travel. Creeks and rivers connected to ocean inlets or other bodies of water will, as they flood current into your channel, or suck it out, cause the direction nd strength of your current to change from location to location as you move toward that bridge. So over-reliance on the chartplotter for an ETA can lead to trouble.

Even without inlet interventions, changing currents with the tidal cycle, as happens in many rivers and sounds, can render meaningless an ETA based on your speed over ground. The operator has to factor in information on currents, when they occur, and their force. Some chartplotters display arrows showing the direction and anticipated velocity of the current, but this can only be a very rough guesstimate because of changes made by moon phase, wind, and other factors. Also, we've seen chartplotters displaying information that doesn't reflect the fact that tidal current doesn't necessarily change direction at the very moment of high or low tide. To further complicate it, the change of tide moves up and downstream. It doesn't start running 10 miles up the river at the same time it starts running at the river mouth.

One Aid To Navigation

Perhaps the most serious deficiency of the chartplotter is its inability to do what you alone can and must do: Look around. Once I was a passenger on a very fine yacht. Its owners were taking it to the Bahamas for the first time. They had excellent navigation equipment. We entered via the North Rock Channel, north of Bimini. There is reef on all sides as you follow (hopefully) the deep water. The owner's eyes were glued to the chartplotter. At one point the screen blinked for some reason. There was great consternation because we were in a narrow part of the passage and shallow reef was on both sides.

Actually, there was no problem. All they had to do was look around. You could see the reef perfectly well because the light was good and the water calm. The chartplotter was helpful, but not really necessary.

Chartplotters can make boating easier, more fun, and safer. They are a remarkable aid to navigation, but that's all they are. Remember, never rely on one aid to navigation. Don't let your reliance on the chartplotter make you lose familiarity with your other navigation tools. 

Tom Neale, a technical and lifestyle writer and liveaboard cruiser, leads our distinguished "Ask The Experts" team.

— Published: February/March 2015

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