Chartplotters are powerful extensions of GPS technology and provide tremendous convenience, but they can get you into trouble if you’re not careful. While my focus is on vector-based plotters (digitized charts, which are the ones typically found on recreational sailboats), most of my observations also apply to raster-based (scanned charts) plotters.
Scale change matters. One of the nice things about using a chartplotter is that you can rapidly change scales by zooming in or out. However, you need to be careful when you do this because not everything will scale proportionally. A good example is cross-track error. When the scale is zoomed out, a boat will appear to be very close to the track line. But when you zoom in closer, it can become clear that, in fact, the boat is actually far away from the track line (Fig. 1). The reason for this optical illusion is that the size of the boat icon is constant, regardless of the display scale. In this case, the proper solution is to constantly monitor the numerical value of cross-track error.
Understand the small-screen effect. Although their sizes do vary, a typical government paper chart will be approximately 1500 square inches and commercial paper-chart books will be about 400 square inches. In contrast, a chartplotter that has a 7-inch diagonal screen will have a display of less than 24 square inches. One result of this size differential is that chartplotters have to make significant compromises when they display information. If, for example, all the information from a paper chart were to be packed onto a small screen, the display would become unreadable.
You could use the declutter or similar feature on the chartplotter to omit details, but this might remove potential navigational hazards. Or you could display just a small portion of the area, the big picture. More-sophisticated plotters have a split-screen function that lets you simultaneously view a zoomed-in image for details and a zoomed-out image for the big picture. But it still isn’t an ideal solution because instead of seeing one 24-square-inch display, you are now seeing two 12-square-inch displays.
The logical conclusion is that unless your chartplotter is very large—as large as the units aboard a commercial ship—it should not be the sole source for your navigational planning and tracking. Rather, you should use paper charts as well. Of course, a paper chart might not contain all the necessary detail either; small-scale charts, which cover larger areas, generally have less detail, while large-scale charts, which cover smaller areas, tend to have a lot more detail. Therefore, the preferred procedure when working with paper charts is to use small-scale charts to keep track of the big picture and large-scale charts for more detail and precise position-fixing.
A chartplotter can’t think for you. Chartplotters are sophisticated, but they don’t understand what they are displaying. A chartplotter will give you the distance and bearing to a waypoint, but it won’t comment on whether there are any hazards along the way (Fig. 2). While most navigators wouldn’t let such a blunder occur, it happens more often than you may think. For example, if you establish a safe waypoint but are then set off the rhumb line by current, the chartplotter may alert you to a significant cross-track error, but it won’t know, and therefore won’t tell you, that your present course is potentially dangerous (Fig. 3). In fact, the plotter might even suggest another dangerous course to get you back onto the track line. Technology is being developed that will address this problem, but it’s not yet available. Until it is, my suggestion is that you keep a paper chart handy so you can keep track of the big picture.
A shoal can move. It might be reassuring to see a nice electronic image of all the hazards surrounding you on the plotter: “Shoals ahead? No problem—all we need do is thread our way through them.” The problem is that shoals (especially ones that are made of sand) can move. If you’re transiting through such an area, you shouldn’t rely exclusively on a chartplotter to keep you clear of the bottom. This is the time to use as many additional navigational inputs as possible—visual observations, compass bearings, depth readings, and the like. And be sure you have installed the most current chart chips available.
Update your chips. This is essential because it’s not just shoals that can move. Buoys can be repositioned, numbers can be changed, and new hazards, such as, wrecks, might emerge. Most chip manufacturers have subscription update services. These can be cheaper than making individual chip purchases.
A chartplotter won’t display everything. No matter how current your chips are, don’t expect your plotter to display all the potential hazards around you—another boat for example.Sailors who previously wouldn’t go out in fog and other low-visibility situations are doing so now because they have a chartplotter. Although a chartplotter is a big help in these situations, you must also keep a sharp visual lookout.
Don’t assume the plotter is correct. Several months ago, a driver who was dutifully obeying his car’s GPS made a right-hand turn—right onto some railroad tracks and in front of an oncoming train. He got out of the car in time, but the car was wrecked. While you could argue that he should have seen what was ahead of him, the incident is a great reminder that electronic devices shouldn’t be followed blindly. You need to use as many other navigational inputs as you can, including your eyes.
Plan ahead. You should always try to plan your passages in advance. Looking at both paper and electronic charts, selecting the waypoints, and then building your routes before you get under way is the best way to minimize the chance of making a plotting error.
I know there is a lot of discussion of whether it is necessary to carry paper charts if you have a chartplotter. Some commercial ships don’t have to carry paper charts, but their chartplotter displays are extremely large, similar in size to a conventional paper chart. Those ships also have a lot of equipment redundancy. Large plotters aren’t feasible for most recreational users, which means most of us are going to be using small chartplotters. Some of you may not agree, but I believe strongly that you could be vulnerable if you rely exclusively on just a small chartplotter. Personally, I won’t get under way unless I also have appropriate paper charts for the passage I am about to make.
Chartplotters have helped revolutionize some aspects of navigation, but navigational errors still occur and are sometimes the very tools that are intended to prevent them. Almost any problem can be minimized by having a solid understanding of the characteristics and the limitations of both GPS and chartplotters. You should also continue to use other electronic aids—a depth sounder, for example—and always keep up your other navigational skills: piloting, dead-reckoning, and, yes, using a paper chart.
Larchmont, New York-based Steven Henkind, M.D., Ph.D., is a marine-safety consultant who also teaches navigation.
